Symposium CH02—Solid-State Chemistry of New Materials

Sunlight-driven photocatalysis is an environmentally friendly approach to solve ecological issues and energy crisis. The development of simple, yet sufficiently stable, photocatalytic materials responsible for harvesting full-spectrum light remains a significant challenge so far. With regard to the design of advanced photocatalysts, low-dimensional materials show tremendous prospects due to their unique surface and electronic properties, which are not seen in their bulk counterparts. Among the various inorganic semiconductors, copper sulfides (Cu_2-xS, 0 ≤ x ≤ 1) have attracted great interest in applications linked to photovoltaic devices and photocatalysis due to their low cost, high element abundance, non-toxic nature, structural and compositional versatility.¹ However, the properties of copper sulfides are sensitive to the Cu-S stoichiometry, which determines the density of copper vacancies, thus the hole concentration; therefore it is critical to finely tune their phases for use in applications.
Herein, we first demonstrate the phase transformations of bulk copper sulfides from digenite (Cu₉S₅) to djurleite (Cu_1.97S) and low-chalcocite (Cu₂S) by the continuous reactive thermal annealing in ambient pressure chemical vapor deposition (CVD), followed by their top-down exfoliation. XRD, HRTEM, and XPS analyses confirm that monoclinic Cu₂S is primarily formed at higher temperatures or greater reaction times and using reducing atmosphere owing to its lower free energy of formation and greater symmetry in its crystal structure. Although copper sulfide does not have a typical MX₂ structure common for transition metal chalcogenides (TMCs), we successfully exfoliated it into 2D flakes using high-frequency liquid-phase exfoliation (LPE) due to its layered morphology.² From AFM, we measured the average thickness of approximately 4 nm of the exfoliated flakes with relatively large lateral size up to 10 μm. We tested the three phases of bulk copper sulfides and their exfoliated forms as photocatalysts for dye degradation under simulated solar light irradiation. Exfoliated Cu₂S flakes exhibited superior photocatalytic activity with a rate constant roughly twice higher than that of bulk chalcocite, which could be predominantly attributed to their unique 2D structure and layered morphology, where the exfoliated flakes can provide an ultrahigh fraction of surface atoms, which could serve as the active sites for dye photodegradation. In addition, the enhanced photocatalytic activity could be attributed to the 2D planar defects such as stacking faults in different portions of exfoliated Cu₂S that were introduced during the annealing in reducing atmosphere without altering its structure and crystallinity. Furthermore, optical spectroscopy results demonstrate that the 2D Cu₂S has sufficient bandgap energy for simultaneously absorbing across the whole solar spectrum. Photocatalytic mechanism experiments and electron-spin resonance (ESR) data indicate that both superoxide radicals and holes served as dominant active species during the photocatalytic reaction. Our study provides a better understanding of possible phase transformations in the copper sulfide system and paves the way for developing next-generation 2D copper sulfide photocatalysts to capture a broad range of the solar spectrum for environmental decontamination.

References
(1) Coughlan, C.; Ibáñez, M.; Dobrozhan, O.; Singh, A.; Cabot, A.; Ryan, K. M. Compound Copper Chalcogenide Nanocrystals. Chem. Rev. 2017, 117 (9), 5865–6109.
(2) Telkhozhayeva, M.; Teblum, E.; Konar, R.; Girshevitz, O.; Perelshtein, I.; Aviv, H.; Tischler, Y. R.; Nessim, G. D. Higher Ultrasonic Frequency Liquid Phase Exfoliation Leads to Larger and Monolayer to Few-Layer Flakes of 2D Layered Materials. Langmuir 2021, 37, 15, 4504-4514.

Molten phases of metal–organic networks offer exciting opportunities for using coordination chemistry principles and reticular synthesis to access liquids and glasses with unique and tunable structures and properties. Here, we discuss general thermodynamics strategies to design extended coordination solid structures with an increased enthalpic and entropic driving force for reversible, low-temperature melting transitions and illustrate this approach through a systematic study of novel bis(acetamide)-based networks with record-low melting temperatures. The low melting temperatures of these compounds are the result of weak coordination bonds, conformationally flexible bridging ligands, and weak electrostatic interactions between spatially separated cations and anions, which collectively reduce the enthalpy and increase the entropy of fusion.
Through a combination of crystallography, spectroscopy, and calorimetry, we demonstrate that trends in enthalpy of fusion are dictated by the strength of coordination bonds and hydrogen bonds within each extended coordination solid, while trends in entropy of fusion are strongly influenced by the degree to which residual motion and positional disorder are restricted in the crystalline state. Pair distribution function (PDF) and extended X-ray absorption fine structure (EXAFS) analysis of Co(bba)₃[CoCl₄] [bba = N,N′-1,4-butylenebis(acetamide)], which features a record-low melting temperature for a three-dimensional metal–organic network of 124 °C, provide evidence of metal–ligand coordination in the liquid phase, as well as intermediate- and extended-range order that support its network-forming nature. Glass formation is observed from the metal–organic liquids with various chemical compositions, and rheological measurements are used to rationalize differences in glass-forming ability and relaxation dynamics. These findings reveal exciting potential of manipulating melting thermodynamics of extended coordination solids by controlling structural and chemical factors in their crystalline state, as well as accessing metal–organic glass and liquid phases with unique structures and properties.

Modulation of electronic properties as a function of external stimuli is driven by multifunctional device development. For instance, optical control over material electronic structures offers a powerful approach for optical switch integration, memory device evolution, and photocatalyst development. Tailoring electronic properties of metal-organic frameworks (MOFs) was previously achieved through metal node engineering, redox-active linker installation, or guest incorporation. We coupled previously applied synthetic strategies in this research area with the possibility of dynamically modulating electronic properties through integration of photoresponsive moieties within a framework.¹⁻³ In this presentation, a photophysics–electronics correlation will be discussed as a function of external stimuli. Optical cycling, modeling of density of states near the Fermi edge, conductivity measurements, and photoisomerization kinetics were employed to shed light on the process of tailoring MOF optoelectronic properties. This talk will also cover our recent efforts for engineering the first photochromic MOF-based field-effect transistor, in which the field-effect response could be changed through light exposure. As a demonstration, the change in current upon light exposure was sufficient to operate a two-LED fail-safe indicator circuit.¹

References:
1. Martin, C. R.; Leith, G. A.; Kittikhunnatham, P.; Park, K. C.; Ejegbavwo, O. A.; Mathur, A.; Callahan, C. R.; Desmond, S. L.; Keener, M. R.; Ahmed, F.; Pandey, S.; Smith, M. D.; Phillpot, S. R.; Greytak, A. B.; Shustova, N. B. Angew. Chem. Int. Ed. 2021, 60, 8072–8080.
2. Dolgopolova, E. A.; Berseneva, A. A.; Faillace, M. S.; Ejegbavwo, O. A.; Leith, G. A.; Choi, S. W.; Gregory, H. N.; Rice, A. M.; Smith, M. D.; Chruszcz, M.; Garashchuk, S.; Mythreye, K.; Shustova, N. B. J. Am. Chem. Soc. 2020, 142, 4769–4783.
3. Dolgopolova, E. A.; Galitskiy, V. A.; Martin, C. R.; Gregory, H. N.; Yarbrough, B. J.; Rice, A. M.; Berseneva, A. A.; Ejegbavwo, O. A; Stephenson, K. S.; Kittikhunnatham, P.; Karakalos, S. G.; Smith, M. D.; Greytak, A. B.; Garashchuk, S.; Shustova, N. B. J. Am. Chem. Soc. 2019, 141, 5350–5358.

The construction of host-guest catalyst systems for CO₂ hydrogenation reactions is described wherein molecular ruthenium-based complexes are encapsulated in the metal-organic framework (MOF) UiO-66. Encapsulation of the molecular complexes is made possible by utilizing an aperture-opening methodology that capitalizes on the dissociative ligand exchange process that occurs for the UiO-66 MOF. Through this method, host-guest catalyst constructs can be synthesized that decouple the synthesis of the transition metal complex, which serves as the catalytically active subunit, with the MOF host material. This advantage was used to develop a variety of catalysts used for the hydrogenation of carbon dioxide to useful fuels, such as formic acid and methanol. The method enables multicomponent catalytic systems that benefit from cascade catalysis. Additionally, the method is ammenable for installing functional groups that modulate catalytic function through second-sphere intereactions, which come about through modifications made to the MOF host material.

Our team investigates transport functionality—ion, electron, and mass—using aerogels as an ultraporous mesoscale platform that innately wires in three dimensions these three necessary mobilities for catalysis [1]. We track performance with a model bifunctional catalytic reaction: oxidation of carbon monoxide at metal nanoparticles (NPs) supported on reducing oxides. We can use the flexibility of sol–gel routes to prepare oxide aerogels as a means to site Au NPs either in the covalently bonded oxide network of TiO₂ or supported on the network using conventional routes to oxide-supported metals. We find that Au||TiO₂ interfacial design strongly impacts charge carrier (electron and proton) transport over mesoscale distances in the metal guest–oxide host architecture with implications for catalytic oxidations and controls the activity of CO oxidation under broadband illumination [2]. The intimacy of interfacial contact between Cu NPs supported on either TiO₂ or CeO₂ aerogels maintains the Cu NPs in a sufficiently metallic state to sustain a plasmonic response without ligand stabilization or during exposure to water/O₂ conditions or CO oxidation. The CeO₂ support also markedly increases activity for the thermal catalysis of CO oxidation over that obtained for Cu NPs supported on titania aerogels enabling Cu-based rather than Au-based oxidative catalysis [3].

[1] D.R. Rolison, J.J. Pietron, E.R. Glaser, T.H. Brintlinger, J.P. Yesinowski, P.A. DeSario, J.S. Melinger, A.D. Dunkelberger, J.B. Miller, C.L. Pittman, J.C. Owrutsky, R.M. Stroud, M.D. Johannes, ACS Applied Materials & Interfaces 2020, 12, 41277–41287.
[2] A.M. Pennington, C.L. Pittman, P.A. DeSario, T.H. Brintlinger, R.B. Balow, S. Jeon, J.J. Pietron, R.M. Stroud, and D.R. Rolison, ACS Catalysis 2020, 10, 14834–14846.
[3] C.L. Pitman, A.M. Pennington, T.H. Brintlinger, D.E. Barlow, L.F. Esparraguera, R.M. Stroud, J.J. Pietron, P.A. DeSario, D.R. Rolison, Nanoscale Advances, 2020, 2, 4547–4556

Abstract: Among several classes of porous materials, metal-organic frameworks (MOFs) are particularly attractive due to their unprecedented internal surface areas (up to 7800 m²/g),^[1] easy chemical tunability, and strong, selective binding of a host of guest species. Through judicious selection of MOF building blocks, which include metal ions and organic ligands, one can readily modify their properties for a variety of potential applications. Despite these attractive features, there are still challenges in the field that limit our ability to use MOFs as a solution for a wide range of industrial problems. For instance, some MOFs have limited mechanical and chemical stability, particularly in highly humid, acidic or basic environments. Overcoming this problem could lead to extended lifetimes and hence increased feasibility for their use in areas where such conditions are required.
In response to these needs, we have recently begun to combine MOFs and polymers in an effort to boost MOF performance and extend their stability. Our recent work demonstrates that selected polymers can significantly enhance MOF performance in a number of important liquid and gas separations^[2-4] as well as extend catalyst lifetimes in selected reactions.^[5] In addition to this, controlled polymerization processes were employed to enhance the mechanical stability^[6] of large pore frameworks and extend the chemical stability of a number structurally diverse MOFs not only in humid environments, but also in acidic and basic media.^[7] We hope such work can help bring these frameworks a few steps closer to their deployment into a range of ecologically and economically important applications. In this presentation, our recent work, devoted to the development of novel MOF polymer composites, will be outlined.
References

1. Hönicke, I. M.; Bon, I. S. V.; Baburin, I. A.; Bönisch, N.; Raschke, S.; Evans, J. D.; Kaskel, S. Angew. Chem. Int. Ed. 2018, 57, 13780.
2. Sun, D; Peng, L.; Reeder, W. R.; Britt, D. K.; Moosavi, M.; Tiana, D.; Oveisi, E.; Queen, W. L.; ACS Central Sci, 2018, 4, 3, 349.
3. Sun, D; Yang, S.; Oveisi, E.; Gasilova, N.; Queen, W. L. J. Am. Chem. Soc. 2018, 140, 48, 16697.
4. Peng, L.; Yang, S.; Sun, D; Asgari, M.; Queen, W. L. Chem. Commun. 2018, 54, 10602.
5. Karve, V. V.; Sun, D. T.; Trukhina, O.; Yang, S.; Oveisi, E.; Luterbacher, J.; Queen, W. L. Green Chemistry 2020, 22, 368.
6. Peng, L.; Yang, S.; Jawahery, S.; Moosavi, S. M.; Huckaba, A. J.; Asgari, M.; Oveisi, E.; Nazeeruddin, Md K.; Smit, B.; Queen, W. L. J. Am. Chem. Soc., 2019, 141, 12397.
7. Yang, S.; Peng, L.; Sun, D; Asgari, M.; Oveisi, E.; Trukhina, O.; Bulut, S.; Jamali, A.; Queen, W. L. Chem. Sci. 2019, 10, 4542.

Solid-state framework materials represent a unique class of solids in which chemists are able to exercise fine control of the crystalline structure through a carefully tuned combination of strong, directional bonding interactions and rigid organic struts. This level of control has enabled the design of highly effective materials for uses as varied as ion transport and gas capture and storage. However, despite these achievements, normal liquids remain the media of choice for many industrial adsorption applications as their fluidity greatly simplifies process design. As a means of bridging the liquid and solid adsorption paradigms we are interested in investigating the molten phases of framework materials. Due to their strong bonds, most framework materials decompose prior to melting. The few zeolitic imidazolate frameworks (ZIFs) which have been shown to melt do so at ~400 °C, temperatures too high to observe appreciable physisorptive interactions with guest molecules. We hypothesized that replacing the strong metal-ligand coordination bonds used in many frameworks with weaker hydrogen bonds would lead to significantly lowered melting points.

I will discuss our work on creating hydrogen-bonded frameworks (HOFs) with low melting points. By borrowing anti-crystal engineering strategies from the ionic liquid literature, we successfully lowered the melting point of a guanidinium sulfonate HOF from ~200 °C to as low as 70 °C. In fact, our molten HOF is itself an ionic liquid, albeit a very viscous one. The melting point of the framework is strongly dependent on its interactions with physisorbed guest species and can vary from 70 – 140 °C with different guests. In the absence of guests molecules, the HOF ionic liquid does not crystallize instead vitrifying into a glassy state at 45 °C. This phase behavior is strongly reminiscent of the clathrate hydrates; where water crystallizes into a network of hydrogen-bonded cages with encapsulated non-polar guests at low temperatures and high gas pressures. We demonstrate that the phase diagram of our HOF-guest system is isomorphic with that of a typical clathrate hydrate, emphasizing this connection. We suspect that the clathrate-like behavior we observe in our HOF may be a general phenomenon of molten frameworks at sufficiently low temperatures and this conceptual framework might be useful for future studies on molten framework materials.

Hydrogenations are an integral part of many catalytic reactions. It is also natural to ask, then, whether this reaction can be catalyzed by hydride compounds. In solid state chemistry, the development of many new hydrides and related mixed-anion compounds has been reported; these advances form an excellent basis for developing new catalysts. We are interested in how these hydrides interact with small molecules, and whether they can lead to novel catalytic materials. Often, these are active as bulk materials, even without any supported metal. We will present how these materials can be used for ammonia synthesis/decomposition and CO₂ hydrogenation, while examining how the bulk structure/composition is involved.

Dielectric screening plays an important role in reducing the strength of carrier scattering and trapping by point defects for many semiconductors such as the halide perovskite solar materials. However, it was rarely considered as a screen to find new electronic semiconductors. We performed a material search study using the dielectric properties as a screen to identify potential electronic materials in the class of metal−pnictide ternary sulfosalts, containing Bi or Sb. These salts are basically ionic due to the electronegativity difference between the S and both the metal and pnictogen elements. However, we do find significant cross-gap hybridization between the S p-derived
valence bands and pnictogen p-derived conduction bands in many of the materials. This leads to enhanced Born effective charges and, in several cases, highly enhanced dielectric constants. We additionally find a series of compounds with low effective mass, high dielectric constant, and other properties that suggest good performance as electronic materials and also several potential thermoelectric compounds.

Within the condensed matter science community, interest in topological materials has grown rapidly. An exciting direction is the combination of topological electronic states with additional quantum phases including magnetic topological materials, which have a range of applications including spintronics, thermoelectric and photovoltaics. Previously, our group discovered the trigonal La₂O₃-type Mg₃Bi₂ material to be a type-II nodal-line semimetal. The strong spin orbit coupling (SOC) Bi layers in this structure create an ideal environment for the topological electronic states. To drive this material even further magnetic elements can be introduced, potentially leading to a coupling between the magnetism and topological quasiparticles that can be additionally enhanced through pressure techniques. Widely accepted as a beneficial clean tool, pressure has the ability to tune the electronic interactions in quantum materials through the atomic distances. In this work we explore various La₂O₃-type structure magnetic topological candidates consisting of either 3d or 4f magnetic elements. Various pressure techniques have been utilized to observe the intricate tuning of the magnetic and electronic properties in these materials including high-pressure neutron diffraction, revealing evident nuclear and magnetic structure transitions. Manipulating these materials will result in a further understanding of our ability to control the magnetic and topological properties.

Since the ferroelectric (FE) property was first reported in the fluorite-structured Si-doped HfO₂ thin film in 2011, numerous researches on this emerging material have been conducted for memory and energy-related applications. The FE characteristic appears in the HfO₂ thin film because of the formation of the polar orthorhombic (o) phase. Although the non-polar monoclinic (m) phase is a thermodynamically stable phase in bulk HfO₂, it has been reported that the o-phase can be formed in HfO₂ thin films by controlling various factors such as dopants, surface energy, mechanical encapsulation, oxygen vacancies, and cooling rate. Meanwhile, the chemically reactive TiN bottom electrode (BE) has a tendency to induce a chemical reaction with the growing HfO₂ film, which can cause defects in the film and produce an unwanted interfacial dielectric layer. The former could significantly degrade the reliability through a leakage current flow and the eventual breakdown of the film during the endurance cycle tests, whereas the latter may degrade the remanent polarization (P_r) by inducing the depolarization effect. In this regard, the nitride interfacial layers, especially HfN_x (or HfO_xN_y) and ZrN_x (or ZrO_xN_y), are noteworthy. The inclusion of nitrogen may suppress the unwanted chemical between the (Hf, Zr)O₂ (HZO) film and the TiN BE, which could improve the reliability of the HZO film and suppress the depolarization effect. Among the HfN_x and ZrN_x, HfN_x could be more promising because it may suppress the in-situ crystallization of the HZO film during its atomic layer deposition (ALD), which adversely interfere with the o-phase formation.
In this study, the enhancements in the FE performance of the HZO-based TiN/HZO/TiN device with the insertion of HfO_0.61N_0.72 interfacial layer (IL) were examined in detail. After optimizing the growth condition of the HfO_xN_y layer by the reactive sputtering, a 2-nm-thick HfO_0.61N_0.72 IL was deposited on the TiN BE, which was oxidized in-situ to the HfO_1.62N_0.34 during the subsequent ALD of the HZO thin film. The adoption of the 2-nm-thick HfO_0.61N_0.72 IL suppressed the uncontrolled growth of larger HZO grains during the subsequent rapid thermal annealing process, which was accompanied by a more than 60% decrease in the inclusion of the undesirable m-phase compared to the structure without the IL. This improvement resulted in a ~30% increase in 2P_r without any change in the coercive field. Such improvement was maintained up to the HZO thickness of 20 nm. This could be ascribed to the formation of an amorphous HfO_1.62N_0.34 IL on the TiN BE, which suppressed the unwanted in-situ inclusion of the crystalline nuclei in the HZO film during the ALD of the HZO film. Also, the slight inclusion of the nitrogen atoms at the bottom interface enhanced the reaction barrier property against the adverse chemical reaction with the reactive TiN BE, which crucially improved the electrical reliability of the HZO film. As a result, the 10-nm-thick HZO film showed an endurance cycle of 1.5×10¹¹ cycles with a 2P_r value of 23.3 µC cm^-2 when the endurance cycle test was performed at 3.0 MV cm^-1. This is among the highest endurance performance levels even without cation doping. Therefore, HfO_0.61N_0.72 IL was highly promising in various FE HfO₂-based devices.

Owing to the interesting spin gap behavior, low-dimensional magnetic systems attract considerable attention. Transition metals, such as Cu²⁺ and V⁴⁺ with spin S = ½, are often used to construct such systems. Recently, we synthesized two crystalline compounds which contain both Cu²⁺ and V⁴⁺ ions by the hydrothermal pouch method. Single crystal X-ray diffraction reveals that triangular “CuV2” motifs with different arrangements forms in the infinite one-dimensional chain compound [H₃N(CH₂)₂NH₃]Cu(H₂O)₂[V₂O₂F₈] (1) and the molecular tetramer compound [Cu(H₂O)(4,4’-dimethyl-2,2′- bipyridyl)]₂[V₂O₂F₈] (2). Magnetization measurement in a pulsed high-magnetic field suggests a presence of a spin gap of 48.6 and 57.6 K in compound 1 and compound 2, respectively. First-principles calculations indicate the presence of strong and weak antiferromagnetic interactions between Cu²⁺ and V⁴⁺, and weak ferromagnetic interactions between V⁴⁺ and V⁴⁺ in both systems. As a result, the quasi-one-dimensional chain compound 1 can be regarded as a chain of weakly coupled linear S = ½ spin trimers, and the molecular tetramer compound 2 can be decoupled into two weakly coupled S = ½ spin dimers.

Next generation materials of nearly every kind rely on chemical, electronic, and/or magnetic heterogeneity for creating, harnessing, and controlling functionality. High entropy oxides (HEOs), materials exhibiting a single-phase crystal structure containing five or more different cations on single crystallographic lattice sites, have attracted great interest in diverse fields because of their inherent opportunities to tailor and combine materials functionalities. The control of local order/disorder in the class is by extension a grand challenge towards realizing their vast potential. We present detailed investigation of the cation site preferences, chemical short range order, and magnetic and thermal properties achieved in specific transition metal-based manganite perovskite and ferritic spinel HEO families through compositional tuning and variation in synthesis/processing conditions. A combination of local to long-range electron, X-ray and neutron scattering probes are employed to investigate their complex configurational diversity and associated structure-property trends. Experimentally derived models are supported by Density Functional Theory calculations and Metropolis Monte Carlo simulations. This work hints at the exquisite level of detail that may be required in computational and experimental approaches to guide structure-property tuning in emerging HEO materials.

A family of high entropy oxides with the formula Mg₂Ta₃Ln₃O₁₄ (Ln = La, Pr, Nd, Sm, Eu, Gd) has been discovered and synthesized. Single crystals, 5 mm OD × 2.5 cm, for Ln = Nd have been grown using the laser optical floating zone technique. Crystal orientations are confirmed by Laue diffraction, and structure solutions were obtained via single crystal X-ray diffraction. The structure is found to be a partially disordered pyrochlore, space group Fd-3m, fractional chemical formula (Mg_0.25Nd_0.75)₂(Mg_0.25Ta_0.75)₂O₇. Magnetization measurements indicate ordinary paramagnetic behavior in all compounds down to T = 2 K, except in the Eu variant which possesses Van Vleck paramagnetism. Specific heat measurements for Ln = Nd shows no phase transitions between T = 300 and 2 K. We demonstrate the ability to prepare magnetically disordered materials by substitution of Mg with Ni, Mn, and Co, demonstrating the flexibility of this family in accommodating defects. The stabilization of these compounds could be due to the entropy gain associated with defects, showcasing a “materials by design” approach by using disorder to stabilize novel magnetic and optical materials. Further work has been done targeting high entropy versions of other families of oxides, with particular focus on the perovskite structure.

Recently, changing political and environmental demands are being met by an intense growth of research on efficient, environmentally friendly and ecological technologies. Among these technologies, lead-free ferroelectric ceramics have been widely studied, as possible applications range from the substitution of lead-based materials in piezoelectric devices and innovative cooling technologies to energy harvesting and conversion applications. However, their full potential remains unexploited for both bulk materials and thin films, the latter being a major driving force in today’s key technology sectors. In this context, the ferroelectric potassium sodium niobate (KNN) ceramic is assumed to bear great potential ever since its discovery in the 1950’s due to its combination of material properties i.e., its perovskite structure and correlated first-order phase transformations and ferroelectricity, as well as its high Curie temperature. However, composition and microstructure control are major challenges due the material’s multi-scale chemical heterogeneity, including high volatility, as well as varying diffusion constants and leading to an insufficient electrical performance as compared to lead derivatives.

The focus of this work is on the analysis, optimization and theory-guided synthesis of KNN, based on the geometrically nonlinear theory of martensite (GNLTM). The latter gives precise rules for the minimization of structural fatigue, but its applicability to complex ceramic materials is unclear. For KNN a major goal is the suppression of abnormal grain growth and the bimodal grain size distribution that lead to degradation of thermal and electronic properties. We show that the GNLTM correctly predicts favorable lattice structures and thus improved material properties for the homogeneous ceramic in which abnormal grain growth has been successfully suppressed. We achieve excellent thermal stability and repeatability of the tuned ceramic as seen from temperature-dependent X-ray diffraction (XRD), temperature-dependent transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). These investigations reveal that the GNLTM together with advanced synthesis and characterization techniques provides a useful strategy for the improvement of the reversibility and functionality of phase-change materials with complex chemistry and structure.

Metal heteroanionic photocatalysts have gained attention for their visible-light absorption; however, they are also plagued by photocorrosion, which limits their long-term use. Such photocorrosion occurs from the photooxidation of the less electronegative non-oxide ions, leading to decomposition of the material’s lattice. In this presentation, new intergrowth oxyhalide photocatalysts composed of tantalum and gadolinium will be shown to be stable materials under visible light for photocatalytic water splitting. These intergrowths were prepared using a halide flux method, with their crystal structures analyzed by Rietveld refinement of powder X-ray diffraction data and scanning transmission electron microscopy. These analyses support intergrowth formation. The Ta:Gd molar ratio was systematically varied in the intergrowths to rationalize the effect of charge separation and changes in band structure towards photocatalytic water splitting activity. Furthermore, these intergrowths are capable of sustained overall water splitting in a Z-scheme with Ru/SrTiO₃:Rh as a hydrogen evolution catalyst. The high stability of these intergrowth materials is attributed to O 2p orbitals at the valance band edge rather than Cl 3p orbitals, as discerned from electronic structure calculations. These results provide new strategies for designing durable artificial photosynthetic systems by rational modulation of crystal and electronic structure.

In pursuit to obtain water-stable MRI contrast agents, the bidentate ligand picolinamidine (PiAm) was used to make complexes with copper, nickel, and zinc. The bidentate coordination of PiAm is analogous to that observed in 2,2′-bipyridine, but PiAm has an additional nitrogen atom which can act as both hydrogen bond donor, and acceptor, helping to improve the aqueous solubility of the target complexes. In this work seven different complexes with one, or two PiAm ligands, as well different geometries were obtained with Cu^IICl₂, Cu^IIClBr, Ni^IICl₂, Ni^II2H₂O and Zn^IICl. Interestingly these complexes are water-soluble, and they can be promising for some applications such as aqueous polymerization, hydrogen splitting, and MRI contrast agents. The stability of these complexes in water was tested using two different approaches: (i) using UV-vis, and (ii) by recovering the initial complex from an aqueous solution after the evaporation of water. Both approaches lead us to conclude that PiAm remains coordinated to the metal ion in an aqueous environment, although in some cases the chlorine ligands can be displaced. The geometry, chemistry, and similarities or differences in the crystal packing of these complexes will be discussed in detail in this work. Finally, the accurate measure of solubility and the EPR data of these complexes are underway and will be presented during the congress.

Nickel-based heterogeneous catalysts are widely researched as cost-effective alternatives to Pt or Au based materials for many reactions related to carbon chemistry, with ongoing efforts focused on improving stability and specific activity. Dispersing Ni on or within a highly porous oxide – such as Al₂O₃, TiO₂, or CeO₂ – can improve performance by both exposing more active sites and promoting metal-support interactions. However, the surface chemistry and stability of the active Ni phase is often unclear, as is the potential effect of the supporting oxide on the presence and persistence of certain phases. In this work, we photodeposit nickel nanoparticles (NPs) onto CeO₂ aerogels, an architecture that provides high surface area and a mesoporous structure allowing facile permeation of gas to interior active sites. Through ex-situ XPS analysis, we show that intimate contact between Ni and CeO₂ stabilizes Ni as Ni(OH)₂, the highly active phase for CO oxidation, Conversion to NiO that occurs in higher weight loading with aggregated Ni is prevented. Activity for Ni/CeO₂ aerogels is far greater than that of either unsupported Ni or Ni supported by structurally similar TiO₂ aerogels, indicating that this system benefits from a unique interaction between CeO₂ and Ni rather than simply the high surface area of the aerogel. CeO₂ aerogels with 2.5 wt.% Ni have both higher activity per mol of Ni and greater stability than higher weight loadings, with 95% CO conversion at 200 °C and no decline in activity after 12 h of continuous reaction. Our results reveal CeO₂ aerogels serve as an ideal supporting oxide for Ni catalysts and provide important insights regarding the effect of Ni phase for CO oxidation.

Conversion reaction synthesis (CRS) methods provide a scalable and versatile route to nanoporous metals (NPMs) of both noble and non-noble metals with a morphology that is highly desirable for catalysis and other applications that benefit from a high surface area. CRS-derived NPMs can exhibit enhancements in catalytic activity relative to conventional metal nanoparticles, though the optimal surface area will be system-specific and represents a compromise between the demands for activity (higher surface areas are better) and stability (which the highest surface area morphologies may lack). For this reason, it is important to gain both a fundamental understanding and practical control over processes that can be used to tune particle and pore sizes in NPMs.

While thermal annealing is generally suitable for tuning porosity, it has been observed that CRS-prepared NPMs often collapse during heating, resulting in the loss of surface area and particle interior accessibility. We have found that the collapse can be avoided by heating not the pure NPMs, but instead heating the NPMs when they are in the form of a CRS intermediate nanocomposite phase in which salt (typically LiCl) fills the NPM pores. In situ synchrotron X-ray diffraction studies with fine time resolution (∼1 sec step size) have been used to quantitatively follow the rapid evolution in microstructure that occurs when Fe, Co, and Cu NPM-salt nanocomposites are heated. Intriguingly, the most rapid growth in the metal particle size occurs concurrently with a sharp reduction in lattice strain within the salt phase, indicating a non-innocent role of the solid salt in the coarsening process. This work provides the first quantitative mechanistic insights into the coarsening behavior of CRS-prepared NPMs.

Work has also been carried out to extend the CRS methodology from simple homogeneous metal systems to more complex heterogeneous systems that can have improved catalytic properties. One example of this is a ZnO-supported Co catalyst, which has an advantageous anisotropic morphology that provides better access to the interior porosity of the nanoporous Co metal than traditional isotropic NPM morphologies. It was found that the combination of the ZnO support and the anisotropic morphology led to a 3x improvement in H₂ production during the steam reformation of ethanol relative to chemically analogous isotropic nanoparticles.

Microwave interaction in materials has been studied, debated, and extensively used for its exciting effects in the last two decades. Applications for microwave processing have grown over the years, especially for material sintering, waste processing, organic and inorganic material synthesis, and the rate enhancement of chemical reactions.
The intrinsic dielectric loss in a material can be explained by lifetime broadened two-phonon difference processes. At temperatures near phase transition, this absorption due to phonon relaxation can result in different structural orientations of the crystal leading to order-disordered phases and phase retention in the crystal structure.
In this work, we investigate the effect of microwave radiation on structural phase transitions and present the results for the case of bismuth oxide. Bismuth oxide has numerous applications in solid oxide fuel cells, gas sensors, photocatalyst, and optoelectronics for its attractive properties of wide bandgap, ionic conductor, and high refractive index. There have been five polymorphs of Bi₂O₃ reported so far, including α, β, δ, γ, and ε, with α-Bi₂O₃ being the stable phase at room temperature. However, under microwave radiation, we were able to synthesize the metastable β phases at room temperature. The sample is characterized using X-ray diffraction, Raman spectroscopy, Differential Scanning Calorimetry, and UV-Vis spectroscopy to evaluate the structure and different phases in the sample. These metastable states are structurally related, and different phases can emerge depending on the temperature and intensity at which the sample is irradiated with the microwave. Lone pair activity plays a crucial role in the crystal structure, and the position and direction of these lone pairs explain the orientation of the crystal. The underlying electronic phenomena can be attributed to the co-operative pseudo-Jahn teller effect, which is the interaction of active pseudo-Jahn teller centers. By evaluating the symmetry descent in the crystal lattice for the phases adopted by bismuth oxide through heat treatment, one can justify the hypothesis of a co-operative effect. As phonons carry the interaction between these centers, we hypothesize that the intrinsic microwave absorption leads to multi-phonon relaxation, disrupting the interaction between the centers. To calculate the intrinsic microwave absorption and support the hypothesis, we evaluate the phonon dispersion and the two phonon relaxation using first-principle calculations.

The materials science community is constantly striving to discover new critical materials that meet future technological demands. Exploring rare-earth free magnetic materials is one direction, where Mn-based compounds have been identified as a promising candidate to replace the rare-earth elements. Here we have designed and synthesized Mn-based novel rare-earth free magnetic materials in the anti-CeCoIn₅ structure type by using chemical bonding concepts. Physical property measurements have confirmed that the magnetic behavior of these compounds range from room temperature ferromagnetic to an antiferromagnetic. Furthermore, theoretical assessments indicate that by including a strong spin orbit coupling and manipulating the lattice parameters, in particular the c/a ratio, a magnetic transformation from antiferromagnetic to ferromagnetic state or vice versa can be observed. Various pressure dependent techniques are being utilized to investigate this predicted behavior. Overall, these materials provide an ideal platform to enhance the understanding of intricate structure-magnetic relationships. Fundamental knowledge on the manipulation of magnetic properties will accelerate the discovery of new critical materials.

Bimetallic structures are advantageous to reduce the use of precious metals and diminish the cost of catalysts for critical chemical reactions. For instance, Pd is used for alkyne hydrogenation, Ru for the Fischer-Tropsch process, and Pt is an excellent catalyst for CO or methanol oxidation. However, these metals are expensive, and monometallic particles are not always very stable. To this end, Pt, Pd, and Ru are often incorporated with Cu to create optimized intermetallic or core-shell structures. The combination of these three expensive metals and Cu also yield a unique electronic configuration at the surface of the particles. Alloying leads to increased stability, activity, and selectivity compared to monometallic compounds. However, dynamical restructuring effects during chemical reactions may alter the property of the bimetallic nanocatalysts. Change in valence states, composition, and crystallography will influence the physical properties of innovative alloys for catalysis.

In this study, three Cu-based alloys were synthesized and characterized. In-situ transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) were performed to understand structural and compositional changes upon exposure to elevated temperatures and O₂/H₂. Modification of the crystal structure at the surface of the nanoparticles and changes in the oxidation numbers can be correlated with increased or reduced chemical properties. Differences between alloyed and monometallic systems are investigated with EELS and electron pair-distribution function (ePDF) analysis. Results are also supported with density functional theory (DFT) calculations to understand the effect of crystallography, valence state, and composition on reactions.

Hence, bimetallic nanocatalysts' chemical and physical properties can be described by a combination of innovative characterization techniques supported by electronic structure calculation. This approach can be expanded to an extensive range of new materials to understand the origin of their properties.

Designing efficient and stable water splitting-photocatalysts is an intriguing challenge for energy conversion systems. We report on the optimal fabrication of perfectly aligned nanotubes on trimetallic Ti-Mo-Fe alloy with different compositions prepared via the combination of metallurgical control and facile electrochemical anodization in organic media. The X-ray diffraction (XRD) patterns revealed the presence of composite oxides of anatase TiO₂ and magnetite Fe₃O₄ with better stability and crystallinity. With the optimal alloy composition Ti- (5.0 at.%) Mo- (5.0 at.%) Fe anodized for 16 h, enhanced conductivity, improved photocatalytic performance, and remarkable stability were achieved in comparison with Ti- (3.0 at.%) Mo- (1.0 at.%) Fe samples. Such optimized nanotube films attained an enhanced photocatalytic activity of ~0.272 mA/cm² at 0.9 V_SCE, which is approximately four times compared to the bare TiO₂ nanotubes fabricated under the same conditions (~0.041 mA/cm² at 0.9 V_SCE). That was mainly correlated with the emergence of Mo and Fe impurities within the lattice, providing excess charge carriers. Meanwhile, the nanotubes showed outstanding stability with a longer electron lifetime. Moreover, carrier density variations, lower charge transfer resistance, and charge carriers dynamics features were demonstrated via the Mott-Schottky and electrochemical impedance analyses.

Engineered materials and devices typically involve interfaces of various forms that introduce property modifications of interfaced materials by introduction of stresses and strains. In this study, we focus on interfaces created in silicon associated with inhomogeneous stresses and specifically address their strong effect on radiative properties of indirect bandgaps. In particular, attention is drawn to the sol-gel based dielectric coatings catalyzed by various acids on silicon that modulates the indirect bandgap of Si favorably towards radiative recombination of free-carriers. A hypothesis on the role of random nanoscale stresses is presented. Photoluminescence and strain levels properties of surface engineered interfaces via colloidal coatings is compared to those of thermal and chemical deposited coatings. Appreciable improvements of band edge light emission is revealed. Other techniques and structures that results also in inhomogeneous bulk strains and expected efficient radiative recombination properties will be presented. One of these include formation of ordered nanoscale dislocation loops below the silicon surface.

Long-term viability of Li-ion battery energy storage may be limited by global lithium reserves. Recently, sodium-ion batteries (NIBs) have garnered a great deal of attention, partially because of sodium’s natural abundance, its more even distribution across the earth and it costs less than lithium. NIBs are especially attractive for large-scale energy storage systems where cost and life cycle are essential. To this end, Na₂FePO₄F has been a material of interest as a viable cathode material for sodium-ion storage due to the high discharge voltage and favourable theoretical capacity.¹ Recent work has shown the potential for doping in Na₂FePO₄F cathodes to increase average voltage, reduce band gap and Na-ion diffusion barrier.²
In this work we will present work completed on computationally investigating Na₂FePO₄F using Density Functional Theory (DFT) with the periodic code, VASP (Vienna Ab Initio Simulation Package), and the hybrid HSE06 ³ (Heyd-Scuseria-Ernzerhof) functional. Here we elucidated the bulk geometric structure of the material through relaxation as well as the most stable magnetic configuration. The electronic structure of the material has also been examined with a Density of States (DOS) and band structure being obtained. Furthermore, to examine the defect landscape of the material properly later, the different stable phases of Na₂FePO₄F have been investigated. Calculating the phase stability of Na₂FePO₄F builds a foundation for a detailed understanding of its defect chemistry at given chemical potentials.

References
B. L. Ellis, W. R. M. Makahnouk, Y. Makimura, K. Toghill, L. F. Nazar, Nature Mater, 2007, 6, 749-753
D. Jin, H. Qui, F Du, Y. Wei, X Meng, Solid State Sciences, 2019, 93, 63-69
A. V. Krukau, O. A. Vydrov, A. F. Izmaylov, G. E. Scuseria, J. Chem. Phys., 2006, 6, 224106

Physicists and chemists share a common belief. The properties of solids are determined by the arrangement of its constituting atoms, which is governed by the chemical bonding mechanism present in the material at hand. It should therefore be possible to revert this relation and infer from a material’s properties which bonding mechanism is employed. This is no simple feat, as there is a multitude of physical properties which might be an indicator for the relevant bonding mechanism in the corresponding solid. We therefore utilize means of computer science by using classification algorithms to sort a database of materials to a priori arbitrary groups, which we aim to link to bonding mechanisms.
In our study, we have composed a database of elements and binary compounds, containing for example Aluminum, Gallium Arsenide, Diamond, Sodium Chloride, Germanium Telluride etc. For all compounds in the database, the corresponding values of a set of properties were included. This set contains the conductivity σ, the Born Effective Charge Z*, the Effective Coordination Number (ECoN) and the Bandgap EG, the melting point T_M, the density ρ and the atomic density ρ_A. To further increase the size of the dataset, miscible compounds were alloyed and the parameters interpolated accordingly, resulting in a total of about 330 compounds used for the classification.
A Gaussian mixture algorithm was utilized to separate the compounds in the database into n clusters, where each value from 2-6 was tested for the number of allowed clusters n. For three clusters, the traditional separation into covalent, ionic and metallic compounds could be reproduced, as no compounds with Hydrogen bonds or Van-der-Waals bonding were added to the database. Only few materials were wrongly classified, with the majority being mix-ups between covalent and ionic compounds. Interestingly, with the number of allowed clusters n set to 4, exactly those materials that have recently been proposed to utilize a novel chemical bond, named metavalent bond, are being assigned to the fourth cluster. We hence conclude, that our results support the concept of metavalent bonding by an a priori computational method. More importantly, this classification provides a close link between chemical bonding mechanisms and properties enabling novel routes to materials design.

Metal-Organic Frameworks (MOFs) have been of great interest for decades due to their porosity and wide application possibilities, such as gas sorption, separation, catalysis, luminescence sensing, and biomedical applications. Among them, MOFs have been utilized as materials with antimicrobial properties against microorganisms. In this work, our group synthesized a Cu-MOF containing glutarate (Glu) and 4,4’-azopyridine (azpy) by solvothermal method. We tested its antifungal activity against two fungi, C. albicans and A. niger, and found that the Cu-MOF shows great antifungal properties for these two fungi. Based on this antifungal ability, it is expected that the material we synthesized can be used for purposes such as food storage and antifungal agents used for various contamination and infection. Detailed results and mechanisms will be discussed.

With more than 20% of the world’s electricity currently used for cooling and a growing population demanding more comfortable living conditions amid an ever warming global climate, the need for sustainable cooling technologies is expected to increase dramatically over the next several decades. Most commercial cooling systems rely on vapor-compression cycles of environmentally harmful refrigerants that are prone to leakage. As an alternative, caloric effects can drive solid-state cooling cycles that feature a high energy efficiency and zero direct emissions, as well as faster start-up times, quieter operation, easier recyclability, and greater amenability to miniaturization than conventional vapor-compression technologies. Here, we will describe how pressure-induced thermal changes—barocaloric effects—mediated by order–disorder or spin-crossover phase transitions can be leveraged for solid-state cooling. In particular, we will discuss the structural and chemical factors that contribute to entropy changes, sensitivity to hydrostatic pressure, and thermal hysteresis in a series of metal–organic materials that exhibit large barocaloric effects.

Despite their high theoretical specific energy of 2,600 Wh kg^-1, the commercialization of Li-S devices is hindered by irreversible capacity loss from the dissolution of polysulfide intermediates in the electrolyte solution. Inspired by the rich chemistry in solid-state materials, we integrate molecular units capable of tethering polysulfides into metal-organic frameworks (MOFs) to improve cycling stability and charge conductivity. Incorporation of redox-active moieties in the framework further enable fast charge and discharge capabilities. Identification of structure-property-function relationships in tunable molecular platforms provide a pathway for translatable strategies across materials types and battery technologies.

Crystallographically determined long-range ordered structures of materials are often taken for granted when investigating properties and functionalities. Although this simplifies our lives, there is a growing awareness of the fact that short-range deviations from the long-range order is not only common but also determining for many of the properties, such as bandgap and magnetic order. As an example, many oxides with the perovskite structure, ABO₃, have well-defined crystalline order, but their properties can in some cases be explained better if one accounts for the local disorder.

To be able to understand how properties are affected by the short-range deviations, or local structure, we first have to properly describe what that local structure looks like. Just as with long-range ordered crystal structures, we can determine the local structure using scattering experiments and subsequently fitting the data to models. To obtain the local structure, however, we must also analyze the diffuse scattering in addition to the Bragg scattering, which is best done in real space as the pair distribution function (PDF). We build on the many already existing PDF studies on perovskites and constrain our structure model further by taking advantage of the knowledge that the structure tends to distort in ways that keep its BO₆ octahedra intact. The resulting distortions, collective or not, can therefore often be described by one of 23 unique tilting patterns of the rigid octahedra, identified by Glazer in the '70s [1]. In our implementation, fitting a model of any of the tilting patterns to scattering data requires refinement of no more than four variables at once – one lattice parameter and a maximum of three tilt amplitudes. Here we present our PDF analysis of a selection of perovskites and determine their degree and type of structural tilting disorder.

PDF analysis and modeling in the Billinge group was supported by the U.S. National Science Foundation through grant DMREF-1922234.

[1] Glazer, A. M. ‘The Classification of Tilted Octahedra in Perovskites’. Acta Cryst. B 28, no. 11 (1972): 3384–92.

Transition metal compounds based on the structurally simple rutile structure exhibit a range of collective electronic phenomena, including metal-insulator transitions, superconductivity, and nontrivial fermions. The ability to manipulate such electronic states through structure is key to realizing new classes of adaptive and low-power microelectronics. Here I briefly overview our progress in understanding the structural contributions to driving metal-insulator transitions in binary and ternary rutile and trirutile compounds using electronic structure based methods. I will first describe our understanding of how geometric and bonding constraints determine the crystallization of compounds in the trirutile structure as opposed to other ternary structures in this space. Here, we predict several previously unreported candidate trirutile oxides, and one is successfully synthesized. I will then present results on vanadium-free rutile and trirutile phases, focusing on the stability and transformations in the electronic structure of 4d and 5d compounds where strong electron-electron interactions are expected to be weak. Our main finding is that one can control the conducting or insulating state by structural design through inductive control over metal-metal interactions with multiple cations or anions. Last, I will describe our recent progress in utilizing this structure-function understanding to automate the discovery process of electronic transitions with Bayesian optimization techniques.

This work was performed in collaboration with Ram Seshadri, Stephen Wilson, Emily Schueller, Kyle Miller, Jaye Harada, Lauren Walters, and Danilo Puggioni and financially supported by the National Science Foundation under awards DMR-1729489, DMR-1729303, and DMR-2011208.

Current solar absorbers for thin film technologies (e.g., CIGS or perovskites) can show good efficiencies above 20% but show limitations in terms of toxicity, earth-abundance or long-term stability. Finding new solar absorber is a cumbersome process involving complex synthesis and characterization steps. Computational modeling using density functional theory (DFT) on the other hand offers an attractive way to speed up this process. Here, we will report on a large scale high-throughput computational search for new solar absorbers among known inorganic materials. The originality of our approach lies in the consideration of intrinsic defects that are essential in controlling carrier lifetime.[1] More precisely, we searched among 7,000 copper-based known inorganic materials for materials presenting adequate band gap, transport (effective masses) and intrinsic defects (i.e., defects that will not act as important non-radiative recombination centers). We will report on a series of candidates and identify the copper phosphides and especially the alkali copper phosphides as very promising potential new solar absorbers. We will present the compounds of interest which have been synthesized before but have never been considered for solar applications and discuss the intrinsic chemical reasons for their adequate PV properties.

[1] Dahliah D., Brunin G., George J., Ha V.-A., Rignanese G.-M., Hautier G., “High-throughput computational search for high carrier lifetime, defect-tolerant solar absorbers”, Energy Environ. Sci., accepted (2021)

Due to the low atomic mass and high phonon frequencies of hydrogen, hydride compounds under extreme pressures are most promising in the search for near room temperature superconductors. First-principles based computational search has become extremely important not only predicting new materials but also guiding high-pressure experimental measurements. However, the calculation of electron-phonon coupling and the corresponding T_c for a given system is usually computationally very expensive, limiting the application of computational search to a few numbers of candidate systems.

In this work, we have developed a super-efficient and fast high-throughput method for searching high-T_c hydride superconductors. We introduce new "metrics" that are strongly correlated to strong electron-phonon coupling and T_c but it is much faster to calculate them. Using our new method, we have searched more than 100,000 binary hydride superconductors and discovered many new high-T_c systems. Among them, we report our prediction of high-temperature superconductivity at relatively low pressure of 50 GPa in a novel binary metal hydride which may break the current record. A detailed mechanism of the superconductivity, phonons and electron-phonon coupling, anharmonicity, as well as the abnormal T_c-pressure dependence, will be also discussed. Our work will not only greatly accelerate the discovery of new hydride superconductors but also give a detailed understanding of important factors that yield superconductivity near room temperature at high pressures.

Knowledge of the entropy change at a phase transition remains critical for engineering materials. For allotropic phase transformations in transition metals and congruent transitions in intermetallic compounds, computation of the electronic structure, and by extension the entropy, has remained particularly difficult. We introduce a formalism that links electronic transport properties with the state electronic entropy to circumvent these difficulties in compounds like Cu₃Au and transition metals including Fe and Co. Our formalism connects materials chemistry and the partial molar entropy of electrons in solution, opening the door for quantitative engineering of the phase transition entropy via chemistry. Application to allotropic phase transitions in some transition metals at atmospheric pressure suggest a possible electron-driven origin of hexagonal close packed stability.

Sodium batteries emerge as an alternative to produce the new, so called, post-lithium batteries (1). In this study, we explore (i) the effect of sodium content and sintering temperature in solid electrolytes based in NASICON-type compounds and (ii) the use of two methodologies to obtain porous NASICON samples: application of natural substances and organic materials as pore-formers and freeze casting. The main purpose is the attainment of hybrid quasi-solid state electrolytes (2), with enhanced room temperature conductivity, based on porous ceramic electrolyte layers infiltrated with ionic liquids. Using this approach, porous samples with different microstructure and porous morphology and distribution were achieved, providing an enhancement in conductivity (ranging from 0.45 to 0.96 mS cm-1 at 30°C) of one order of magnitude for infiltrated samples respect to pore-free samples. Preliminary electrochemical test on thick electrode showed a specific capacity higher than 80% respect to the theoretical one, which is quite promising considering the thickness of the electrolyte. According to these results the porous NASICON might be considered as a functional macroporous inorganic separator that can act as a Na+ reservoir.
The authors would like to thank the Agencia Española de Investigación /Fondo Europeo de Desarrollo Regional (FEDER/UE) for funding the projects PID2019-106662RBC43. This work has also been supported by Comunidad de Madrid (Spain) - multiannual agreement with UC3M (“Excelencia para el Profesorado Universitario” - EPUC3M04) - Fifth regional research plan 2016-2020.
References
[1] N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Research development on sodium-ion batteries, Chem. Rev. 114 (2014) 11636–11682
[2] M. Keller, A. Varzi an S. Passerini, J. Power Sources, 392, (2018), 206-225

Site-directed and robust chemical binding of the nanocrystalline surface to a substrate is of utmost importance in the fields of printable electronics, fluorescence assays and catalysis, among others. Historically, nanocrystal preparation methods have involved chemically inert ligands than are post-synthetically exchanged from the surface of the crystal—whether in colloidal solution of in the solid film—to a more useful and/or fitting molecule for the given application. Here, in line with the overarching theme of the Niezgoda lab, we present our findings on the development of a preparation for CdSe quantum dots (QDs) prepared with a native terminal alkyne ligand, 10-undecynoic acid (UDYA), as the lone surface-bound molecule. Further, core/shell architectures of these particles have been prepared with decent quantum yields in the visible spectrum. High-resolution TEM images with elemental mapping show well-formed and monocrystalline particles, and the presence of the alkyne on the surface is verified with FTIR and NMR spectroscopies. These first-of-their kind native alkyne-coated fluorescent QDs are immediately useful in alkyne-azide click chemistry applications, wherein the QDs can be site-directed to any substrate decorated with azide moieties. Examples of colloidal, thin film and 3D-printed systems involving alkyne-azide click cyclization of the novel QDs to substrate are shown as proof-of-principle demonstrations of the wide applicability of these particles. We emphasize that the fundamental innovation of this project is that it effectively circumvents the need for a post-synthetic ligand exchange to accomplish robust QD-substrate chemical linkage, while still providing QDs with suitable quantum yield and stability.

The development of materials used for electrodes brings advances in energy storage technology. In addition, research on how to maximize the efficiency of electrodes with a combination with solid-state electrolytes, one of the key elements of energy storages. Polymer-based electrolytes such as solid polymer electrolytes (SPEs) and gel polymer electrolytes (GPEs) are of great interest due to their flexibility in creating flexible energy storages. It also solves the weakness of liquid electrolytes: liquid leakage, volatility, flammability, weak chemistry properties, and electrochemical properties. Herein, we demonstrate a way to improve the electrochemical performances of the supercapacitor (SC) by infiltrating GPEs with high ion conductivity into electrode made of multi-walled carbon nanotube (MWCNT). GPEs are synthesized through polymerization of poly(ethylene glycol) methyl ether acrylate (PEGMA) in the presence of trimethylolpropane ethoxylate triacrylate (ETPTA) in LiPF₆ organic solution. By changing the chemical composition of PEGMA, ETPTA, and lithium solution we can adjust the mechanical properties and ionic conductivity of GPEs. Electrode-infilling gel polymer electrolyte (EI-GPE) with high ionic conductivity improves the characteristics of the electrode-electrolyte interface and creates a high-performance SCs. The SCs produced in this way exhibit excellent performance in capacitance and energy density, and exhibit a long cycle life even under mechanical deformation.

Structure-property connections are a key theme in heterogeneous catalysis such as photocatalytic water splitting. Bismuth-based layered perovskite compounds such as Bi₄MO₈X (M=Nb, Ta; X=Cl, Br) have emerged as promising durable photocatalysts for visible-light driven water splitting. The unique electronic structure of these oxyhalides in the valence band suppresses the self-oxidation of non-oxide anions by photogenerated holes. The unique combination of electronic and optical properties in these oxyhalide perovskite materials lies in their rich structural complexity. In this study, the electronic and nanoscale structure of multimetal oxyhalide intergrowth oxychloride: Bi₄TaO₈Cl-Bi₂GdO₄Cl have been systematically investigated. We have varied the amount of Bi₄TaO₈Cl in the intergrowths to rationalize the effect changes in the local crystal and electronic structure. A combination of Ultraviolet Photoelectron Spectroscopy, UV-Visible Diffuse Reflectance Spectroscopy and Electron Energy Loss Spectroscopy techniques were used to determine and investigate the electronic structure of the oxyhalide intergrowths. The obtained conduction and valence band energy levels elucidate the possibility of photocatalytic water splitting as well as carbon dioxide reduction. By virtue of aberration-corrected scanning transmission electron microscopy and converged-beam electron diffraction (CBED), the nanoscale structure of the intergrowth materials was elucidated with atomic displacements at an accuracy of several picometers. Based on the high-order Laue zone reflections of CBED patterns, the local symmetry and tilting of TaO₆ octahedra in the intergrowth crystal structure were revealed. We believe our approach demonstrated here opens up opportunities for understanding the atomic-scale structure-property phenomena of novel photocatalytic systems.

Due to the need for small-scale electronic devices, many studies are in progress on energy storage devices. In particular, in the case of microsupercapacitors (MSCs), they can be embedded in small electronic devices and are being used as energy storage devices that can temporarily supply power to rechargeable batteries. For this reason, in this study, MSCs are fabricated by laser photothermal method of 3D porous laser-induced graphene (LIG) electrodes. A fluorine-based polyimide (fPI) film is used as substrates for LIG electrode (LIG-fPI). During the graphitization process caused by the laser, gaseous products including fluorine are released from fPI to form a microporous structure, which can overcome the disadvantage of a small electrode specific surface area seen in conventional LIGs, and increasing the specific surface area is a way to improve the energy density of MSCs. As the electrolyte used in this study, we choose an organic gel polymer electrolyte (o-GPE). MSCs with o-GPE are mechanically flexible and have excellent electrochemical performance. For o-GPE, poly(ethylene glycol) methyl ether methacrylate (PEGMA), trimethylol propane ethoxylate triacrylate (ETPTA) are used, and 2-hydroxyl-2-methyl-1-phenyl-1-propanone (HMPP) is used as a photo-initiator. UV-cured o-GPE impregnated with a liquid electrolyte containing lithium salt is made into a pouch cell with LIG-fPI, which is flexible and bendable. It exhibits very high energy and power densities of 0.58 mW cm^–2 and 0.01 mWh cm^–2, respectively, at a high voltage window of 3 V. The resulting LIG-fPI-MSC is promising for use in rechargeable power supplies in various applications such as wearable microelectronics and the Internet of Things.

Nanomaterials as zinc oxide have potential applications in nanomedicine, biosensors, catalysts and biomedical bio-imaging because of their intrinsic optical properties. We report the synthesis and characterization of zinc oxide nanostructures in presence of ethylene glycol and the effect of the zinc precursor on the particle. Zinc oxide nanoparticles were produced in presence of ethylene glycol and zinc precursors at 197 °C by using a reflux system. The reaction time was ranged from two to six hours and the precursors evaluated were zinc sulphate heptahydrate (II) and zinc acetate dehydrated (II). As-synthesized nanostructures were characterized by Absorption and Photoluminescence spectroscopies. Besides, the presence of ethylene glycol on the ZnO was corroborated by Infrared Spectroscopy. Results evidenced that nanostructures exhibited strong emission peaks at 400 nm which comes from the recombination of their pars electron-hole. Whereas the absorption spectra exhibited shoulder peaks at approximately 360 nm which is attributed to the band gap. Also, preliminary studies evidenced that ZnO nanostructures have catalytic properties to destroy contaminants in aqueous matrices.

Recent work analyzing the impact of non-symmorphic symmetries on electronic states has given rise to the discovery of multiple types of topological matter. Here we report the single crystal synthesis and magnetic properties of EuGa2Sb2, an Eu-based antiferromagnet structurally consisting of pseudo-1D chains of Eu ions related by a non-symmorphic glide plane. We find the onset of antiferromagnetic order at TN = 8 K. Above TN; the magnetic susceptibility is isotropic. Curie-Weiss analysis suggests competing ferromagnetic and antiferromagnetic interactions, with peff = 8.1μB as expected for 4f7 J = S = 7/2 Eu2+ ions. Below TN and at low applied magnetic fields, an anisotropy develops linearly, reaching χ⊥/χ∥ =6 at T = 2 K. There is concomitant metamagnetic behavior along χ∥, with a magnetic field of μοH ≈ 0.5T sufficient to suppress the anisotropy. Independent of crystal orientation, there is a continuous evolution to a field polarized paramagnetic state with M=7 μB/Eu2+ at μοH=2T as T→0 K. Specific heat measurements show a recovered magnetic entropy of ΔSmag ≈ 16.4 J.mol-1.K-1 from T ~ 0 K to T = TN, close to the expected value of Rln(8) for an S = 7/2 ion, indicating little low dimensional spin fluctuations above TN. We find no evidence of unusual behaviors arising either from the dimensionality or the presence of the non-symmorphic symmetries.

Tons of pollutants from textile industries are lost to effluents every year due to the inefficiency of dyeing processes. In this regard, conventional techniques as adsorption or coagulation have been used in water treatment to concentrate pollutants by transferring them to other phases. Alternative destructive techniques are needed to obtain a fast destruction of pollutants in wastewater treatment. In this way, photo-catalysis is an advanced oxidation process that includes a quick oxidation of the pollutant molecules, lack of production of polycyclic products, and final degradation of contaminants in the ppb range. This technology is based on the activation of nanomaterials (i.e. semiconductors) in presence of electromagnetic radiation and subsequent generation of radicals from water to directly oxidize many organic pollutants. Based on the above considerations, this research is an attempt at the use of Zn-based nanomaterials (i.e. as ZnS and ZnO NPs) for the degradation of environmental pollutants from aqueous matrices. The specific goals of this project are: i) Generate and characterize nanomaterials with photoactive properties, ii) Study the photochemical stability of photoactive nanomaterials, and iii) Demonstrate the degradation of environmental pollutants in presence of photoactive nanomaterials and the reuse of the nanostructures. Optical properties showed a luminescent peak at 420 nm and 442 nm and an absorption peak at 270 nm and 307 nm. Photodegrading capacities of quantum dots were evaluated in presence of Acid Red 14 at different concentrations (250 ppm and 500 ppm). Bare ZnS nanomaterials degraded Acid Red 14 successfully (75%) after 180 minutes.

Theoretical and experimental study of inorganic crystals is an interdisciplinary subject that is attracting intense research and development due to both fundamental and applied scientific value. Metallic selenites present different chemical structures and advanced physical properties such as pyroelectricity, ferroelectricity and piezoelectricity. Also, selenium-silver based matrices exhibit interesting structural, optical, and photoluminescent properties, which accounts for potential materials in many research fields like physics, chemistry and materials science. Moreover, the chemistry of selenium and silver is very prominent in biological systems, mainly because of their interactions with molecules like proteins. In this work, silver selenate (Ag₂SeO₄) and silver selenite (Ag₂SeO₃) microcrystals were prepared by the sonochemistry (SC), ultrasonic probe (UP), coprecipitation (CP), and microwave assisted hydrothermal (MH) methodologies. X-ray powder diffraction patterns results showed intense and sharp peaks related to the orthorhombic Ag₂SeO₄ and monoclinic Ag₂SeO₃ structures, attesting that materials are crystalline and present structural long-range order. It was also observed experimental Raman modes for both phase, which corroborated to the theoretical ones calculated indicating structural short-range order. The morphologies were confirmed by FE-SEM microscopy and Wulff construction, showing distinct shapes of different width and length due to the different methodologies of synthesis. Optical properties were attested by UV-vis spectroscopy which showed the characteristic semiconductor band gap value for both materials. Photoluminescence spectroscopy showed an intense band in the visible region with maximum at around 700 nm for all samples. Further trapping experiments prove that the holes and hydroxyl radical, in minor extent, are responsible for the photocatalytic degradation of dyes. The geometry, electronic properties of the bulk, and surface energies of these materials were evaluated using first-principles quantum mechanical calculations based on the density functional theory (DFT/B3LYP) to provide information at the atomic level. In this sense, experimental results were correlated to theoretical studies and enabled a deep understanding between structure and property. Inspired by the above considerations, these materials may present superior characteristics for new compounds with photoluminescent and photocatalytic applications.

Semiconductor-based photocatalytic systems have been frequently used for small molecule activation and transformation reactions, such as water splitting for H₂ and O₂ evolution, CO₂ reduction, and N₂ reduction. During photocatalysis, light-irradiation on semiconductors leads to the generation of excited electrons which compete to recombine with co-generated excited holes, resulting in lower reaction yields. The simultaneous utilization of excited electrons and holes, for example via a cross-coupling mechanism, is thus of critical importance for useful organic transformations, such as C-C bond forming reactions. To realize cross-coupling for C-C bond formation, the redox potentials of carbonyls and alcohols must match the reduction (conduction band minimum) and oxidation (valence band maximum) power of the semiconductor, respectively.

In this work, we perform first principles-based density functional theory (DFT) computations to rationally design semiconductors with proper band-edge positions for photochemistry through varying facets and thicknesses, as well as introducing co-catalysts. We model surface slab structures with different facets for several important semiconductors of interest, such as CdS, ZnIn₂S₄, Cu₂ZnSnS₄ and Ag₂ZnSnS₄ (as well as related compounds with replacement or mixing at cation/anion sites), with different thicknesses of the bulk region. Relative electronic band edges are thus computed for multiple pseudo-2D semiconductors as a function of surface termination and thickness, and compared with redox potentials of relevant alcohols, carbonyls, amines and imines. We further study the formation energies of intrinsic point defects as a function of chemical potential conditions and charge states in selected compounds, namely Cu₂ZnSnS₄ and Ag₂ZnSnS₄, and suggest strategies to suppress the formation of harmful anti-site defects. Finally, we model the adsorption of organic molecules on selected semiconductor surfaces, and study photocatalytic coupling reaction mechanisms in the presence of H atoms or added electrons and determine energy barriers using nudged elastic band (NEB) calculations. Complementary experimental synthesis and characterization of ultrathin 2D semiconductor-based photocatalytic systems and the production of ketyl radicals from carbonyls are carried out by our collaborators. The computational screening undertaken in this work reveals novel ways of tuning the photocatalytic properties of semiconductors via modification of slab thickness and surface termination, composition engineering, and defect manipulation.

Thermoelectric materials have attracted considerable research attention in recent decades due to their ability of reversible conversion between heat and electricity. The enhancement of thermoelectric performance is challenging due to the strong interrelation of Electrical conductivity and Seebeck coefficient. In this work, we studied eco-friendly Cr-doped Cu₂SnS₃ with CuCrSnS₄ nanoparticles. Our experiments and theoretical calculation(DFT) proved that doping Cr atom in Sn site of Cu₂SnS₃ enhanced electrical conductivity by modifying electron density of state. Moreover, by DFT calculation, it is found that doping Cr atom in Sn site makes chemical bonding soften resulting in lattice thermal conductivity decrease. Using XRD analysis, the solubility limit of Cr impurity in Cu₂SnS₃ is around 5 at%. Doping Cr atom above the solubility limit induced the spinel CuCrSnS₄ nanoprecipitations. The superparamagnetic fluctuation of spin angular momentum of spinel CuCrSnS₄ results in exchange interaction with itinerant carrier spin. The synergetic effect of Cr doping made the power factor increase from 1.12 to 6.67 μW/cm/K². The CuCrSnS₄ were verified that second phase in Cu₂SnS₃ matrix by XRD and TEM analysis. The size of CuCrSnS₄ confirmed that 50 nm. Finally, Cr-doped-Cu₂SnS₃ with magnetic nanoparticles showed the highest zT value of 0.8 at 723 K comparing with pristine zT of 0.08 at 723 K.

The solubility of gases in water—despite being relatively low compared to other solvents—is key to many biological and industrial processes. Bulk water's highly structured hydrogen bonding arrangement is thought to impede the formation of transient voids large enough to accommodate gas molecules. However, when water is confined to the nanoscale, such as in a protein cavity, a carbon nanotube, or a microporous material, its structure and properties can differ dramatically from those observed in the bulk. In particular, the solubility of gases in confined water can be at least an order of magnitude higher than in bulk water, possibly due to differences in the density and hydrogen bonding arrangement of confined water. Understanding the origin and extent of this increase in gas solubility is important for biological and industrial gas-dependent processes that involve water, but experimental investigation of the structure of confined water within micropores and its impact on gas solubility is limited. Microporous materials such as metal–organic frameworks and zeolites provide excellent tools to study the structure and gas solubility of water confined to nanoscopic dimensions due to their chemical and structural tunability and well-studied gas adsorption behavior. This work examines the impact of pore size, shape, and chemistry on the structure and gas solubility of confined water, with particular emphasis on solid-state features such as the relative hydrophilicity of pore walls and the presence of open metal coordination sites capable of binding water molecules.

The generation of small Cerium Oxide Nanoparticles (CeO₂ NPs) was performed by a simple precipitation method in the presence of CETAB (hexadecyltrimethylammonium bromide), cerium nitrate hexahydrate ((Ce(NO₃)₃.6H₂O)) and ammonium hydroxide solution (NH₄OH). An amount of cerium precursor was added to CTAB and under constant agitation NH₄OH was added until the pH was at a basic of 10.0. The colloidal solution was heated in a reflux system at the range of 55 to 65 Celsius during 3 hours and 6 hours. The final solutions were purificated using the centrifugation system twice for 15 minutes on 7,000 rpm using deionized water. High resolution transmission electron microscopy (HRTEM) studies evidenced mainly spherical-shape and highly crystalline structures for both samples and average sizes of 6 nm and 8 nm for synthesis of 3 hours and 6 hours, respectively. The presence of CTAB as surfactant material would be controlling the growth and particle size of the material. Electron diffraction (ED) patterns evidenced cubic structures for both sizes in the (111), (200), (220), and (311) reflections which are indexes as face centered cubic (fcc) arrangements. Studies of Energy dispersive X-ray Spectroscopy (EDX) indicated Ce (25.6%) and O (74.4%) in the synthesis of 6 hours, and Ce (29.2%) and O (70.8%) at the synthesis of 3 hours. Generated nanoparticles because of their small sizes and high surface area will be evaluated in photodegradation studies to destroy organic pollutants in aqueous matrices.

Materials based on ceria (CeO₂) show promising degradative activity towards a variety of pollutants and toxins including organophosphorus pesticides or chemical warfare agents. CeO₂ is highly reducible, leading to oxygen vacancy defects that assist in the formation and stabilization of reactive oxygen species (ROS). These persistent reactive species may later degrade toxic compounds over long timescales. We use density functional theory (DFT) to calculate the energies of binding and formation of ROS at multiple types of defect sites on low-index surfaces of CeO₂. Differences in the calculated energies, which should correlate with ROS lifetimes, originate in the geometry and electronic structure of these surfaces. We compare the predicted trends with the spectroscopic characterization of ROS at CeO₂ aerogels and defect-rich Gd-doped CeO₂ aerogels. The combined modeling and experimental results suggest strategies to engineer longer ROS lifetimes for enhanced toxin destruction through controlled nanoparticle morphology and doping.

The total or even partial replacement of copper in the charge reservoir block in the well-known YSr₂Cu₃O_7-d cuprate by other transition metals (TM) allows one to extend the hole doping range with respect to the conventional phase diagram (1). In particular, the introduction of highly valent cations such as TM= Fe³⁺/Fe⁴⁺ or Mo⁵⁺ results in unusually high nominal hole concentrations at the CuO₂ planes (2,3).
Quite interestingly, despite the elevated doping levels, superconductivity with relatively high critical temperatures of Tc = 83 and 70 K are observed in the resulting Mo_0.3Cu_0.7Sr₂TmCu₂O_7+δ and FeSr₂YCu₂O_7+δ compounds, respectively. Besides the realization of unusually high hole doping levels, the chemical modification of the charge reservoir blocks allows one the study of the effect of chemical disorder in the superconducting properties, providing interesting structure-hole doping-Tc relations (4). Also, in the case of the iron by copper substitution, additional complexity arises from the coexistence of magnetic and superconducting interactions (5). Here we present a broad outlook on the unusual properties of these TM substituted cuprates.
References:
[1] S.D. Conradson, T.H. Geballe, A. Gauzzi, M. Karppinen, C. Jin, G. Baldinozzi, W. Li, L. Cao, E. Gilioli, J.M. Jiang, M. Latimer, O. Mueller, V. Nasretdinova, Proc. Natl. Acad. Sci. 117 (2020) 4559–4564.
[2] X.M. de Irujo-Labalde, S.A. López-Paz, S. García-Martín, M.Á. Alario-Franco, J. Solid State Chem. 286 (2020).
[3] S.A. López-Paz, X. Martínez De Irujo-Labalde, J. Sánchez-Marcos, C. Ritter, E. Moran, M.A. Alario-Franco, Inorg. Chem. 58 (2019) 12809–12814.
[4] X. Martínez de Irujo-Labalde, E. Urones-Garrote, S. García-Martín, M.Á. Alario-Franco, Inorg. Chem. 57 (2018) 12038–12049.
[5] S.A. López-Paz, D.P. Sari, A.D. Hillier, M.A. Alario-Franco, AIP Adv. 11 (2021) 015011.

Authors thank Dr. Romero de Paz and Dr. Fernandez-Sanjulian for the magnetic and electric measurements and Dr. Ritter(ILL) for the neutron diffraction measurements. X.M.d.I.-L. thanks MECD for the grant FPU014/05971; S.A.L-P. also thanks MECD for an FPU-016/00732 grant. This work has also been supported by REE (Project No. REE: 44-2014) Fundación Ramón Areces, Projects MAT2016-78362-C4-4-R and MAT2017-84385-R, and CAM project MATERYENER3CM-S2013/MIT-2753.

Fruits reach its maturity several days after its harvest, this process is highly complex since it is associated with biochemical changes that increase the levels of ethylene, which influences its maturation and quality parameters. Ethylene, in its role as an enzyme, is one of the variables with the greatest influence on the ripening and deterioration process in avocado. However, there are also other pathologies and to face pathologies of biotic origin, its management is made with products like imidazole which is toxic and is already banned in some countries. At present, materials such as TiO₂, ZnO, Ag and SiO₂ Nanoparticles could give them the ability to intervene in the amount of free ethylene, and on the other hand they have been found to have antimicrobial activity. TiO₂ nanoparticles are the most widely used at an industrial level due to their photocatalytic activity and properties such as biocompatibility, corrosion resistance and zero toxicity, and Ag nanoparticles due to their photoactivity are promoted as an antimicrobial agents. This work seeks to obtain TiO₂ / Ag nanoparticles by combustion synthesis to search for nanotechnology-based alternatives that can be used in the post-harvest process. Considering the photocatalytic properties of TiO₂ and its relationship with the absorption of ethylene and the antimicrobial properties of Ag, its potential for the postharvest process can be studied. TiO₂ nanoparticles with different concentrations of silver (0%, 0.75%, 1.5%, 3% and 4.5% mole percentage) were obtained by the burning combustion solution method using titanium isopropoxide and silver nitrate as precursors, nitric acid as oxidizing agent and glycine as fuel. Rhodamine-B dye degradation test was performed to verify its photocatalytic activity under visible light and it was observed that indeed the particles obtained have photocatalytic activity. An FTIR study has also been performed on each type of particle and it is observed that there are functional groups such as OH in a peak of 3452.5 cm^-1, which is vital for photocatalysis, there are peaks between 836.8 cm^-1 and 530 , 2 cm^-1 that corresponds to vibrations of Ti-O and there is a peak that can be associated with the interaction of OH with AgNO3 between 1669.3 cm^-1 and 1313.4 cm^-1 since it occurs only in the particles that were synthesized to obtain them with Ag . XRD has been performed to test anatase and rutile with Ag⁺ inclusions in crystallographic phases present after combustion synthesis. As results of this studyAg-TiO₂ particles were obtained by the burning combustion solution method with photocatalytic activity under visible light and having ethylene absorption capacity . The presence of OH, Ti-O groups and the interaction of OH with Ag⁺ can be observed, thereby the presence of silver in the sample and its photocatalytic activity is higher in samples with OH peak is more intense.

Metal trichalcogenides and chalcohalides establishes a class of semiconducting materials with a quasi-one-dimensional crystal structure. These low-symmetry semiconductors have shown favorable optoelectronic properties for photovoltaic and photoelectrocatalysis: an optical bandgap in the visible-NIR region, and a relatively small electron affinity. However, there are limited synthetic strategies for nanostructured chalcohalides. Here we report a facile route for transformation of Sb2Se3 to SbSeI nanorods. The Sb2Se3 nanorods acted as templates to form small diameter SbSeI nanorods. The aspect ratio of SbSeI nanorods could be varied by changing the aspect ratio of the template. A detailed investigation of the transformation process reveals favorable thermodynamics and proceeds through a topotactic mechanism that may be suitable for other trichalcogenide-chalcohalide transformations. These ID materials show good charge transport and a bang gap (Eg) of 1.7 eV, which are suitable for photocathodes for hydrogen evolution reactions.

Non-centrosymmetric intermetallic compounds with antisymmetric spin-orbit coupling exhibit a plethora of emergent properties, such as unconventional superconductivity, topologically non-trivial quantum properties, and quasiparticle behaviour. Chemical bonding preferences in the transition metal silicon-phosphides T-S-P create the asymmetric environment around metal atoms resulting in high abundance of non-centrosymmetric chiral and polar crystal structures. The synthesis of those phases is hampered by inertness of T and Si in contrast to high reactivity and vapor pressure of phosphorus. We developed a comprehensive synthetic strategy allowing to produce single crystals and phase-pure polycrystalline samples of metal tetrel-pnictides. The correlation between the crystal structure, chemical bonding, and optical and transport properties will be discussed.

Zintl phases, with their vast range of structural patterns in non-cubic space groups and their excellent high-temperature thermoelectric performance, stand out as an intriguing subject area for the study of transport anisotropy. The covalently-bonded polyanions exemplified by Zintl phases crystallize in a diverse range of highly anisotropic sub-structures, including isolated moieties, 1D chains, 2D sheets and 3D networks. Despite ample theoretical evidence of light band mass and enhanced thermoelectric efficiency in the covalently-bonded direction in Zintl phases, experimental confirmation is still largely lacking. This knowledge gap is exacerbated by the difficulty of growing single crystals large enough for complete characterization of transport properties. In the present study, we use a combination of theory and experiment to investigate the anisotropic electronic structure, transport properties, and growth habit of Ca₅M₂Sb₆ (M=Al, Ga, In), which is characterized by anionic sub-structures resembling infinite 1D chains of corner-linked MSb₄ tetrahedra. Ca₅M₂Sb₆ single crystals grown from an Sb-rich molten flux were found to form long, needle-like crystals oriented parallel to the polyanionic chains. Due to the small cross-section of the crystals, characterization of transport properties in the direction perpendicular to the chains is challenging. For this purpose, micro-ribbons were extracted from single crystals using focused ion beam milling, and laser photolithography was used to deposit contacts to measure electrical resistivity and Hall coefficients. The resistivity measured parallel to the growth direction was found to be nearly 20 times higher than the perpendicular direction, confirming predictions. In addition, to we investigated the surface energies of various terminations of Ca₅Ga₂Sb₆, revealing that the calcium content plays a central role in surface reconstruction, leading to the formation or breaking of surface covalent bonds. The relative surface energies are a strong function of growth conditions (either Ca rich or Ca poor), suggesting that the flux composition can be used to tune the growth habit, perhaps guiding future crystal growth of Zintl phases.

Compounds in the structure type CaAl₂Si₂, in particular Mg₃Sb₂ and Mg₃Bi₂, have attracted great interest in recent years for their remarkable thermoelectric performance. Their impressive thermoelectric figure of merit (zT) stems, in part, from their anomalously low thermal conductivity. In the present study, in situ high-pressure synchrotron X-ray diffraction was used to investigate the structure and bonding of binary and ternary AM₂X₂ (A=Ca, Mg, Yb; X=Bi, Sb) compounds at pressures up to 50 GPa. These experiments allowed us to develop trends between bond compressibility and cation size and to study the stability of the CaAl₂Si₂ structure type. Our results for binary compounds confirm prior predictions of isotropic in-plane and out-of-plane compressibility but reveal large disparities between the bond strength of the two distinct Mg sites. We show that the octahedral Mg–Sb bonds are significantly more compressible than the tetrahedral Mg–Sb bonds. Conversely, the discrepancy in the polyhedral compressibility for the ternaries is not as pronounced as for the binaries. These results indicate that the bonds of the octahedrally-coordinated Mg are responsible for the ultralow Mg₃Sb₂ and Mg₃Bi₂. Further, we report the discovery of phases transitions above 4.0, 7.8, 11.3, and 18.0 GPa for Mg₃Sb₂, Mg₃Bi₂, YbMg₂Bi₂, and CaMg₂Sb₂, respectively.

Thermal conductivity is the intrinsic property of a material that expresses its ability to conduct heat. The process of heat conduction depends on four basic factors: the temperature gradient, the cross section of the materials involved, their path length, and the properties of those materials. There are two categories of measurement techniques: steady-state and transient. Steady-state techniques infer the thermal conductivity from measurements on the state of a material once a steady-state temperature profile has been reached, whereas transient techniques operate on the instantaneous state of a system during the approach to steady state.
Thermal transport properties of minerals and melts at high pressures and temperatures is of central importance to the evolution and dynamics of planets. The pressure of the Earth’s interior continuously increases with the depth from the surface of the Earth: several hundred MPa at the region of the crust, ∼20 GPa at the upper mantle and ∼130 GPa at the lower mantle. Precise data of thermal conductivity and/or thermal diffusivity for minerals at elevated pressure make it possible to estimate the heat budget in the Earth.
Direct measurements of thermal conductivity of Earth minerals at extreme pressures and temperatures are very challenging; the available experimental data are limited and inconclusive, causing geodynamical studies to rely on theoretical calculations and/or large extrapolations.
In the present work, we demonstrate the optimized technique to perform direct measurements of thermal conductivity of materials in diamond anvil cells at high pressures and high temperature. We used a continuous 100-Watt infrared laser for heating sample from both sides to a desired temperature and a pulsed 300-Watt infrared laser creating a microsecond long heat wave from one sample side. Thermal conductivity can be determined by measuring temperature of sample radiometrically using a streak camera coupled to a spectrograph from both sample sides as a function of time and fitting the results with FE calculations. KCl was used as PTM, Ir was used for absorption of the laser if the absorption of the sample was small. Pulse was shaped to a square shape by a Pockels cell, which simplified and improved the accuracy of FE calculations. Furthermore, we significantly improved FE analysis by introducing automatic procedures based on python codes enabling the fitting parameter optimization. Fast measurements of thermal emissions allowed us to resolve the heating response through the sample enabling direct measurements of thermal conductivity of Earth’s lower mantle minerals and Fe alloys in the core.
This complex system can be used for pressures in the range of 5 to 150 GPa for materials with any laser absorption properties. We used this system to obtain the values of thermal conductivity of ferropericlase, bridgmanite, and iron-silicon alloys which correspond well with literature data albeit mainly measured at room temperature.

Owing to large lattice anharmonicity from interface interactions between adjacent layers, layered heteroanionic materials have the potential to realize ultralow thermal conductivity[1-3]. As an analog of well-known ultralow thermal conductive phase BiCuOSe, BiAgOSe has been predicted to be a better ultralow thermal conductive material by DFT calculations[4,5]. Utilizing both DFT calculations and Pourbaix diagrams[6], we have optimized the hydrothermal reaction conditions and developed an efficient, high yield synthesis to obtain the BiAgOSe phase. Moreover, we have investigated the thermal transport behavior of BiAgOSe, and demonstrated that it has a lower lattice thermal conductivity compared to BiCuOSe. To study the origin of the ultralow thermal conductivity of BiAgOSe, we combined lattice dynamic calculations and synchrotron x-ray diffraction measurements and found that the low lattice thermal conductivity results from strong lattice anharmonicity and low phonon velocities which are associated with the vibrations of the silver ions.

References:
1. Chang, C. et al. 3D charge and 2D phonon transports leading to high out-of-plane ZT in n-type SnSe crystals. Science 360, 778–783 (2018).
2. Zhao, L.-D. et al. Ultrahigh power factor and thermoelectric performance in hole-doped single-crystal SnSe. Science 351, 141–144 (2016).
3. Zhao, L.-D. et al. Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals. Nature 508, 373–377 (2014).
4. Li, J. et al. Predicting excellent anisotropic thermoelectric performance of the layered oxychalcogenides BiAgOCh (Ch = S, Se, and Te). Computational Materials Science 171, 109273 (2020).
5. He, J. et al. Computational Discovery of Stable Heteroanionic Oxychalcogenides ABXO (A, B = metals; X = S, Se, Te) and Their Potential Applications. Chemistry of Materials 32(19), 8229-8242 (2020).
6. Walters, L. N., Zhang, C., Dravid, V. P., Poeppelmeier, K. R. & Rondinelli, J. M. First-Principles Hydrothermal Synthesis Design to Optimize Conditions and Increase the Yield of Quaternary Heteroanionic Oxychalcogenides. Chemistry of Materials 16.

Flexible and lightweight optoelectronic displays depend on luminophores that emit brightly at red, green, and blue wavelengths, but access to the latter is hindered by the limited availability or high cost of phosphors that can produce deep-blue emission. Here, we describe solid-state phosphorescence from a series of tetranuclear copper(I)-pyrazolate (Cu₄pz₄) clusters with high quantum yields and blue emission with color coordinates of [0.15, 0.11]. These emissions are remarkably sensitive to structural effects far from the Cu₄ core: When 3,5-di-tert-butylpyrazoles are used as bridging ligands, adding a C4 substituent can cause a blueshift in photoluminescence by over 100 nm. X-ray crystal structures of several Cu₄pz₄ clusters reveal C4 substitution (H, F, Cl, or Br) to have modest effects on solid-state geometries, yet structure-property trends and time-dependent density functional theory (TD-DFT) calculations show the impact of these remote substituents on phosphorescence to be entirely steric in nature. The size of the C4 unit controls the conformations of adjacent tert-butyl groups and can increase the rigidity of the Cu₄pz₄ metallacycle, which can diminish Jahn-Teller distortion and prevent the excited-state structure from adopting lower energy states prior to photoluminescence. Such remote steric effects on photophysical processes are unprecedented, and may offer a novel approach toward the design of molecular phosphors with tunable optoelectronic properties.

Silver phenylselenide (AgSePh), a two-dimensional (2D) member of the broader class of hybrid organic-inorganic metal organochalcogenide (MOC) family, has gained a renewed interest as a novel class of hybrid organic-inorganic 2D semiconductors. AgSePh features multiple exciting properties such as narrow blue luminescence, in-plane anisotropy, large exciton binding energy, earth abundant elemental composition, and a scalable synthesis. In contrast to 2D layered perovskites and transition metal dichalcogenides, AgSePh has strong covalent bonding between organic and inorganic components. This unique covalent interaction makes AgSePh a truly hybrid organic-inorganic semiconductor and opens a new door to control the properties of 2D semiconductors through organic functionalization. In this presentation, we will show the unique bandgap tuning of AgSePh and its derivatives by systematically introduce para-substituted functional groups with varied electron-donating and electron-withdrawing capabilities. These newly synthesized derivatives adopt 2D layered structures and exhibit strong excitonic characteristics. We will discuss the effect of organic functionalization on the optical and electronic properties, and show the application of this knowledge on designing new MOC compounds with excellent optical properties.

Interaction between lattice defects and dopants in crystalline materials has long been recognized, and its influence on the materials’ properties has been evidenced in a wide range of materials(1). In thin-film semiconductors, significant density of 1D defects, from well-known dislocations to new types of line defect(2), are formed and interact with dopants, and it has received increasing research interest as the interplay of the crystalline defects and solute atoms can alter the electronic and optical properties of the thin films. Despite its significance, the study of the defect-dopant interaction has remained challenging due to the atomic-scale defect structures and low concentration of dopants. Here, analytical scanning transmission electron microscopy (STEM) and ab initio calculations are employed to directly examine the effects of dopants on 1D defects in promising perovskite La-doped BaSnO₃(3, 4).
The local atomic and chemical configurations of two representative dislocations, single (b=a<100>) and dissociated (b=a<110>) dislocations, in La-doped BaSnO₃ thin films are characterized via STEM imaging and energy dispersive X-ray (EDX) spectroscopy. In both single and dissociated dislocations, a substantial amount of dopant segregation inside and outside dislocation cores as well as dopant-driven atomic re-arrangement is observed. Core-loss electron energy-loss spectroscopy (EELS) analysis is used to probe the local chemical and electronic structure variation at the dislocations, and structural optimization and electronic ground state calculations provide insight into the origin of the dopant-induced atomic and electronic modifications. Finally, the dopant behavior at dislocations is compared with dopant segregation observed in unique Sn-core line defects in La-doped BaSnO₃(2), highlighting the complex but predictable nature of dopants at 1D defects and potential applications of these atomic-scale disorders(5).
La-doped BaSnO₃ films were grown on SrTiO₃ and LaAlO₃ substrates by hybrid molecular beam epitaxy method(6). Plan-view STEM samples were prepared by mechanical polishing, and STEM experiments were carried out using an FEI Titan G2 60-300 (S)TEM equipped with EDX and EELS detectors. Ab-initio calculations were performed using the Quantum Espresso package(7) for structural relaxation and WIEN2K code(8) for ground-state calculations.

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Perovskite-inspired materials aim to replicate the optoelectronic performance of lead-halide perovskites, while eliminating issues with stability and toxicity.^1–3 Recently, chalcohalides of group IV/V elements have attracted attention, due to enhanced stability provided by stronger metal-chalcogen bonds, alongside compositional flexibility and ns² lone pair cations — a performance-defining feature of halide perovskites.⁴ A rigorous description of their atomistic properties and performance potential is lacking, however.

Following the first experimental report of solution-grown tin-antimony sulfoiodide (Sn₂SbS₂I₃) solar cells,⁵ with power conversion efficiencies above 4% (exceeding the first reported solar efficiency of methylammonium lead-iodide (MAPI)),⁶ we assess the structural and electronic properties of this emerging earth-abundant PV material.⁷ We find that the experimentally reported centrosymmetric Cmcm crystal structure represents an average over multiple polar Cmc2₁configurations. The instability is confirmed through a combination of lattice dynamics and molecular dynamics simulations. We predict a large spontaneous polarisation of 37 μC/cm² that could be active for electron-hole separation in operating solar cells. The resemblance of this dynamic crystal structure and ferroelectric behavior to that of MAPI begs the question of its importance in high-performance defect-tolerant solar materials.

Moreover, using state-of-the-art ab-initio methods (hybrid Density Functional Theory including spin-orbit coupling effects), we rigorously assess the efficiency limits of this material on the basis of its electronic structure and predicted defect behaviour, calculating η_max > 30 % for film thicknesses t > 0.5 μm.

The results shine a spotlight on the largely-unexplored class of A₂BCh₂X₃ mixed-metal chalcohalides. These are candidates for solution-processed ferroelectric and optoelectronic devices, with the substitutional flexibility for engineering band gaps, band energies, and lattice polarisation. Our work provides insight regarding both the potential success of this emerging class of optoelectronic materials and structure-property relationships in perovskite-inspired materials, guiding design strategies and expanding the compositional space of candidate materials.

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2 Z. Li, S. R. Kavanagh, M. Napari, R. G. Palgrave, M. Abdi-Jalebi, Z. Andaji-Garmaroudi, D. W. Davies, M. Laitinen, J. Julin, M. A. Isaacs, R. H. Friend, D. O. Scanlon, A. Walsh and R. L. Z. Hoye, J. Mater. Chem. A, 2020, 8, 21780–21788.
3 M. Buchanan, Nature Physics, 2020, 16, 996–996.
4 R. Nie, R. R. Sumukam, S. H. Reddy, M. Banavoth and S. I. Seok, Energy Environ. Sci., 2020, 13, 2363–2385.
5 R. Nie, K. S. Lee, M. Hu, M. J. Paik and S. I. Seok, Matter, 2020, S2590238520304471.
6 A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, Journal of the American Chemical Society, 2009, 131, 6050–6051.
7 S. R. Kavanagh, C. N. Savory, D. O. Scanlon and A. Walsh, Materials Horizons, 2021 (Accepted).

Metal heteroanionic materials, such as oxyhalides, are promising photocatalysts in which band positions can be engineered for visible-light absorption by changing the halide identity. Advancing the synthetic routes towards these materials, bismuth oxyhalides of the form BiOX (X = Cl, Br) have been prepared using rapid and scalable ultrasonic spray synthesis (USS). Central to this advance was the identification of small organohalide molecules as halide sources. When these precursors are spatially and temporally confined in the aerosol phase with molten salt fluxes, powders composed of single-crystalline BiOX nanoplates can be produced continuously. A mechanism highlighting the in situ generation of halide ions is detailed. These materials can be used as photocatalysts and provide proof-of-concept of USS as a route to more complex bismuth oxyhalide materials.

Recent years have seen a surge in interest throughout the materials science community in the rational design of crystal structures that maximise a material’s suitability as an organic semiconductor. Predicting the spatial arrangement of molecules in the solid state and, therefore being able to compute the resulting electronic properties, provides insights into structure-property relationships to guide the material design process. We have been predicting polymorphs and charge mobilities for a myriad of [6]helicenes. Helicenes are a class of axially chiral twisted graphene-like molecules, with potential applications as organic semiconductors. Due to their discernible molecular geometry, helicenes tend to pack in recurring structural motifs. The ubiquity of certain motifs across polymorphs can be exploited to screen a vast pool of helicenes for the best charge transport material along a specific motif. We generated approximately 1400 different [6]helicene molecules, packing in the most frequently encountered translational dimer motif. We computed the transfer integral and inner reorganisation energy to screen for good semiconductors. For the most promising [6]helicene molecules, a CSP search was performed and the lowest energy polymorphs’ charge carrier mobility was computed. We observed very low inner reorganisation energies for symmetrically disubstituted [6]helicenes containing side groups with a triple bond. The most promising candidates have been selected for full crystal structure prediction and are being synthesised by our experimental collaborators. More recent work analysing the packing of helicenes on surfaces using coarse-graining methods will also be discussed.

Mixed-valent solid-state compounds have long been known for their vibrant colors, and the pnictogen-halide semiconductors containing ns² lone pairs are no exception, exhibiting intense absorption bands with unusual colors. In contrast, zero-dimensional (0D) ns²-based metal-chlorides without mixed-valency are generally colorless but have recently demonstrated unique properties that are promising for optoelectronic applications such as broadband lighting, thermometry, and radiation detection.
Here, a new family of mixed-valent alkali pnictogen halides with composition Rb₂₃Bi^III_xSb^III_7-xSb^V₂Cl₅₄ (0 ≤ x ≤ 7) is discovered through solvothermal synthesis. Single-crystal X-ray diffraction reveals that the deep red Rb₂₃Sb^III₇Sb^V₂Cl₅₄ crystallizes in an orthorhombic space group (Cmcm) with a unique, layered 0D structure driven by the repulsion of the 5s² lone pairs of the Sb^IIICl₆ octahedra. In contrast to the prototypical Cs₄Sb^IIISb^VCl₁₂ tetragonal structure with highly symmetric octahedral environments, here the Sb^IIICl₆ octahedra exhibit trigonal or disphenoidal distortion from the lone pair, yielding layers with distinct orientations of the octahedra as these asymmetric units pack around the Sb^VCl₆ octahedra. Partially or fully substituting Sb^III with isoelectronic Bi^III yields the series Rb₂₃Bi^III_xSb^III_7-xSb^V₂Cl₅₄ (0 < x ≤ 7), with a similar layered 0D structure but exhibiting rotational disorder that yields a trigonal crystal system with a polar space group (R32). Second harmonic generation of 532 nm light from a 1064 nm laser using Rb₂₃Bi^III₇Sb^V₂Cl₅₄ powder confirms the non-centrosymmetry of this compound. As with the prototypical mixed-valent pnictogen halides, the visible absorption bands of the Rb₂₃Bi^III_xSb^III_7-xSb^V₂Cl₅₄ family are the result of intervalent Sb^III-Sb^V and mixed-valent Bi^III-Sb^V charge transfer bands, with a blueshift of the absorption edge as Bi^III substitution increases. No PL is observed from this family of semiconductors, but a crystal of Rb₂₃Bi^III₇Sb^V₂Cl₅₄ exhibits a high resistivity of 1.0 x 10¹⁰ Ωcm and weak X-ray photoconductivity with a μτ product of 8.0 x 10^-5 cm² s^-1 V^-1. Beyond simply adding a new entry to the growing library of 0D metal halides, the unique 0D layered structures of the Rb₂₃Bi^III_xSb^III_7-xSb^V₂Cl₅₄ family highlight the versatility of the ns² lone pair and offer ripe opportunities for tuning and improving these materials; namely, the ability to tune both lone-pair expression as well as the charge transfer absorption band characteristic of mixed-valent materials within the same structure.

Solar thermochemical (STC) splitting of CO₂ and/or H₂O via two-step metal oxide redox cycles is a promising approach to generate large-scale quantities of synthetic liquid hydrocarbon fuels. STC pathway utilizes the enitre spectrum of solar radiation as a source of high-temperature process heat to split CO₂ and/or H₂O into CO and/or H₂.^[1] The CO and H₂ (syngas) produced may be directly used as a fuel or furthur processed into synthetic liquid hydrocarbon fuels by the Fischer-Tropsch process. The two-step thermochemical temperature-swing cycle involves an endothermic reduction step of a redox metal oxide at high temperatures (1300–1500 °C), generating lattice oxygen vacancies, followed by an exothermic re-oxidation step with CO₂ and H₂O at low temperatures (800–1000 °C).^[2] Perovskite-type (ABO_3-δ) oxides are being considered as an attractive redox material class alternative to the state-of-the-art material ceria (CeO_2-δ), due to their high structural and thermodynamic tunability. However, these advantages are offset by the unfavorable re-oxidation equilibria due to their low entropy change compared to pure ceria, leading to lower mass specific fuel productivity under most conditions relevant for STC cycles. This work reports a novel dual-phase Ce-substituted Mn-deficient perovskite oxide (LSC25M75), La³⁺_0.48Sr²⁺_0.52(Ce⁴⁺_0.06Mn³⁺_0.79)O_2.55 (R-3c, 70 %) and pyrochlore Ce⁴⁺_2.6La³⁺_1.4O_6.28 (Fd-3m, 30 %) composite material, aiming to overcome individual thermodynamic limitations of perovskite oxide and ceria based redox materials. Thermochemical CO₂ redox splitting and long-term cyclability of LSC25M75 evaluated with a thermogravimetric analyzer and an IR furnace reactor over 100 consecutive redox cycles demonstrated a two-fold higher CO₂ conversion extent than the current state-of-the-art perovskite oxide, La_0.60Sr_0.40MnO_3-δ (LSM).
Improved understanding of the true phases and the structure-property relationship of materials under operational conditions is necessary for developing high performance STC redox materials. The underlying reaction mechanism for the obtained high performance were elucidated for the first time using complementary diffraction techniques. Based on in situ high temperature neutron, ex situ synchrotron X-ray and electron diffraction experiments, we obtained an unprecedented structural insight into thermochemical perovskite oxide materials. We propose a novel CO₂ splitting reaction mechanism involving a reversible temperature induced phase transition from the n = 1 Ruddlesden-Popper phase (Sr_1.10La_0.64Ce_0.26)MnO_3.88 (I4/mmm, K₂NiF₄-type) at reduction temperature (1350 °C) to the n = 2 Ruddlesden-Popper phase (Sr_2.60La_0.22Ce_0.18)Mn₂O_6.6 (I4/mmm, Sr₃Ti₂O₇-type) at re-oxidation temperature (1000 °C) after the CO₂ splitting step. ^[3]

References
[1] M. Romero, A. Steinfeld, Energy Environ. Sci. 2012, 5, 9234.
[2] D. Marxer, P. Furler, M. Takacs, A. Steinfeld, Energy Environ. Sci. 2017, 10, 1142.
[3] J. M. Naik, C. Ritter, B. Bulfin, A. Steinfeld, R. Erni, G. R. Patzke, Adv. Energy Mater. 2021, 11, 2003532.

Disorder is commonly used in chemistry for designing functional materials. For instance, preparation of solid solutions is nothing else than the introduction of a controlled number of point defects in a crystal. Disordered systems, though, provide more degrees of freedom: not only the number of defects, but also their distribution can be used to optimise the functional properties of materials, however up until now, defect distribution was hard to control and thus was rarely used in practice.

In this talk we will show how to precisely tune distribution of point defects by changing various chemical parameters during crystal growth, and characterise it with the single crystal diffuse scattering.

We will use Prussian Blue Analogues (PBAs) as our model system. PBAs is a class of cyanide materials with the general formula M[M’(CN)₆]_1-δ * xH₂O where M and M’ are transition metals. Depending on the nature of transition metals, PBAs can accommodate a large number of vacancies on the M’(CN)₆ site (for instance δ=0.33 for M=Mn and M’=Co) which makes them highly porous and, as a result, attractive for hydrogen storage applications. Distribution of M’(CN)₆ vacancies is important for the performance of this material, since more disordered vacancy configurations provide more diffusion pathways through the structure, larger accessible volume, and easier transport.

A recent approach to measure electron radiation doses in the kGy range is the use of phosphors with an irradiation dose-dependent luminescence decay time. NaYF₄:Yb^3+,Er³⁺ is limited as it shows pronounced fading. Therefore, in this work a modified synthesis of three fluorides is presented that the results in nanoparticles co-doped Yb and Er, Ho or Tm that is shown to be a promising dosimeter material. Upon excitation with a 980 nm laser a typical upconversion luminescence is observed. For all these materials a dose dependency of the luminescence decay time is observed. However, of the most importance is the very low fading effect of the luminescence decay time. In the case of SrF₂ that is as low as 5 % in the 30 days after irradiation and beyond. This makes this type of materials attractive for electron beam dosimetry in spite of lower upconversion intensity compared to NaYF₄ or BaYbF₅. In addition all two materials show Na_1.5Yb_1..5F₆ and SrF₂ shows thermoluminescent properties.

Experimental: The precipitation (SrF2) ^[1] is done with an ammonium fluoride solution (all Sigma Aldrich) in distilled water. The hydrothermal syntheses of Na_1.5Yb_1.5F₆:RE run in a PTFE autoclave with a stainless steel envelop. All compounds were characterized by x-ray powder diffractometry and micro structure photography. The samples were irradiated in an EBLab 200 electron irradiator (Comet) using 200 KV acceleration voltage. Doses of 5 kGy up to 50 kGy were applied to the samples. To measure luminescence decay curves, the phosphor samples were excited by a 1 ms light pulse from a 980 nm laser diode (max. 80 mW, Roithner SPL980-100-105M-PD) and the temporal upconversion luminescence decay was recorded by a silicon photodiode (Osram SFH2200). Ten consecutive curves were averaged and then exponentially fitted to obtain the decay time.
Results: After electron beam irradiation (200 keV) of a thin immobilized phosphor layer the luminescence decay time of the SrF₂: 2%Er, 2…20%Yb phosphor decreases from 1520 μs to values between 1250 μs and 1020 μs, depending on the irradiation dose as shown in Figure 1a. For samples irradiated with 5, 10 and 20 kGy, respectively, the luminescence decay times were repeatedly measured until 50 days after irradiation (Figure 1b). The material Na_1.5Yb_1.5F₆:Ho,Yb shows a decay time dependence of the temperature (Figures 2a and 2b) in case of the radiated materials. The altered decay time after electron beam radiation is similar low fading in 30 days than SrF₂;Er,Yb.
Conclusion: In this work, a series of fluoride sub-micron powders co-doped with different amounts of Yb³⁺ sensitizer and trivalent Er, Ho Tm activator ions were synthesized. For SrF₂:Er,Yb(2%/20%), a dose-dependent reduction of the material’s luminescence decay time after electron irradiation is observed. This allows the use of the powder as a dosimeter material. Encouragingly, SrF₂:Er,Yb(2%/20%) shows reduced fading of the luminescence decay time compared to the previously studied NaYF₄:Yb³⁺, Er³⁺. The thermal response of the radiated Na_1.5Yb_1.5F₆:Ho,Yb make this material suitable for temperature supervision ^[2] on electronic boards e.g.

Figure 1: a) end point of the decay time (TCSPC) after different electron beam doses, b) fading of different radiated SrF₂:Er Yb samples (a fifty days storage shows the same)

Figure 2: thermal response of the decay time with radiated SrF₂:Er Yb (a) and Na_1.5Yb_1.5F₆:Ho,Yb (b) by the same dose

[1] Kuznetsov, S.; Ermakova, Y.; Voronov, V.; Fedorov, P.; Busko, D.; Howard, I. A.; Richards, B. S.; Turshatov, A. Up-conversion quantum yields of SrF₂:Yb³⁺,Er³⁺ sub-micron particles prepared by precipitation from aqueous solution. Journal of Materials Chemistry C 2018, 6 (3), 598–604.
[2] Schuster, C.; Kuntz, F.; Strasser, A.; Härtling, T.; Dornich, K.; Richter, D. 3D relative dose measurement with a μm thin dosimetric layer. Radiation Physics and Chemistry 2020 (in submission).

For thermoelectrics doping is usually the first step to tailor the charge carrier concentration and concomitant properties. Doping should also turn GeSe, which features intrinsically a low concentration, into a competitive thermoelectric. Yet, elemental doping fails to adjust the charge carrier concentration. Doping with AgSbSe₂, on the contrary, works and causes a remarkable enhancement of thermoelectric performance. This improvement is closely related to a change of bonding mechanism, as evidenced by sudden alterations of the optical dielectric constant ε_∞, the Born effective charge Z* and the bond breaking mechanism. These property changes signal the formation of metavalent bonds. These bonds lead to an octahedral-like atomic arrangement and a favorable band structure featuring small effective masses. A quantum-mechanical map distinguishes the different types of chemical bonding. It reveals that orthorhombic GeSe is located in a map region where covalent bonding prevails, while cubic and rhombohedral GeSe are located in the region characterized by metavalent bonding. These assignments are in line with the measured properties and thermoelectric performance of the different phases. Our work thus redefines doping rules and provides a ‘treasure map’ to tailor thermoelectric properties of chalcogenides and related p-bonded materials.

Vanadium phosphate positive electrode materials attract great interest in the field of Alkali-ion (Li, Na and K-ion) batteries due to their ability to store several electrons per transition metal. These multi-electron reactions (from V²⁺ to V⁵⁺ ) combined with the high voltage of corresponding redox couples (e.g., 4.0 V vs. Na⁺/Na for V³⁺/V⁴⁺ in Na₃V₂(PO₄)₂F₃) could allow the achievement of the 1 kWh/kg milestone at the positive electrode level in Alkali-ion batteries. However, a massive divergence in the voltage reported for the V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺ redox couples as a function of crystal structure is noticed. Moreover, vanadium phosphates that operate at high V³⁺/V⁴⁺ voltages are usually unable to reversibly exchange several electrons in a narrow enough voltage range. During this talk, through the review of redox mechanisms and structural evolutions occuring upon electrochemical operation of selected widely studied materials, we will identify the crystallographic origin of this trend: the distribution of PO₄ groups around vanadium octahedra, that allows or prevents the formation of the vanadyl distortion (O . . . V⁴⁺=O or O . . . V⁵⁺=O).^1-4 While the vanadyl entity massively lowers the voltage of the V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺ couples, it considerably improves the reversibility of these redox reactions. Therefore, anionic substitutions, mainly O^2- by F^- , have been identified as a strategy allowing for combining the beneficial effect of the vanadyl distortion on the reversibility with the high voltage of vanadium redox couples in fluorine rich environments.

References
1. Boivin, E.; Chotard, J.-N.; Masquelier, C.; Croguennec, L., Towards Reversible High-Voltage Multi-Electron Reactions in Alkali-Ion Batteries Using Vanadium Phosphate Positive Electrode Materials. Molecules 2021, 26, 1428.
2. Boivin, E.; Iadecola, A.; Fauth, F.; Chotard, J.N.; Masquelier, C.; Croguennec, L. Redox Paradox of Vanadium in Tavorite LiVPO₄F_1-yO_y . Chem. Mater. 2019, 31, 7367–7376.
3. Nguyen, L H.B.; Broux, T.; Sanz Camacho, P.; Denux, D.; Bourgeois, L.; Belin, S.; Iadecola, A.; Fauth, F.; Carlier, D.; Olchowka, J.; Masquelier, C.; Croguennec, L. Stability in water and electrochemical properties of the Na₃V₂(PO₄)₂F₃ – Na₃(VO)₂(PO₄)₂F solid solution. Energy Storage Materials 2019, 20, 324-334.
4. Nguyen, L.H.B.; Iadecola, A.; Belin, S.; Olchowka, J.; Masquelier, C.; Carlier, D.; Croguennec, L. A Combined Operando Synchrotron X-ray Absorption Spectroscopy and First-Principles Density Functional Theory Study to Unravel the Vanadium Redox Paradox in the Na₃V₂(PO₄)₂F₃-Na₃V₂(PO₄)₂FO₂ Compositions. J. Phys. Chem. C 2020, 124, 23511–23522.

In cation disordered cathode materials the presence of any short-range ordering of cations can have a strong influence on their function, in particular their electrochemical behaviour on cycling[1]. The recently discovered anti-perovskite cathode material Li₂FeSO has a promising theoretical capacity[2], but still encounters issues with capacity fade and long-term structural degradation[3].

This theoretical study seeks to explore the short range ordering of cations in Li₂FeSO by using a DFT+U based cluster expansion. It will be shown that there is a strong preference for the formation of OLi₄Fe₂ of oxygen octahedra, which we postulate is due to equidistribution of charge. More curiously we find that the asymmetric arrangement of lithium and iron ions around the sulfur and oxygen anions is favoured versus symmetric arrangements. Subsequent Wannier analysis of DFT calculations shows that this stabilisation has its origins in the generation of dipoles on the anions. We conclude that these anion dipoles are not only the origins of the short-range order in Li₂FeSO but also induce disorder in this material.

[1] A. Abdellahi, A. Urban, S. Dacek, and G. Ceder, Chem. Mater. 28, 5373.
[2] K. T. Lai, I. Antonyshyn, Y. Prots, and M. Valldor, J. Am. Chem. Soc. 139, 9645.
[3] M. V. Gorbunov, S. Carrocci, S. Maletti, M. Valldor, T. Doert, S. Hampel, I. G. G. Martinez, D. Mikhailova, and N. Gräßler, Inorg. Chem. 59, 15626.

Superionic conductors (SICs) possess liquid-like ionic diffusivity in the solid state, finding wide applicability from electrolytes in energy storage to materials for thermoelectric energy conversion. Type I SICs, where high ionic diffusivity is linked to the crystal structure via a first-order transition and is therefore either on or off, are attractive for energy storage and conversion applications providing the transition temperature can be controlled. However, such SICs (e.g., AgI, Ag₂Se, etc.) have been found exclusively in 3D crystal structures so far, limiting existing approaches to applied pressure and nanoscale size effects – which affect the transition temperature in one direction – while the effects of chemical substitution cannot be easily decoupled from 3D ion-conducting pathways. Layered materials have spatially separated mobile and immobile sublattices, potentially facilitating wide tunability and functioning as platforms to study two-dimensional superionic conduction.
We previously identified a first-order, order-disorder phase transition in 2D KAg₃Se₂ – a dimensionally-reduced derivative of 3D Ag₂Se – at ~695 K using in-situ XRD and DSC [1]. Here, we use quasi-elastic neutron scattering and AIMD simulations reveal that a-KAg₃Se₂ is a type I SIC with a highly disordered Ag sublattice restricted to 4 Å thick layers [2]. The superionic local structure was probed by in-situ XPDF analysis, confirming the defect dynamics from AIMD simulations . Thermal analyses of cation-substituted AAg₃Se₂ (A = Li-Cs) compounds indicate that the superionic transition temperature can be tuned by the composition of the immobile charge-balancing layers. Future directions to design materials with tailored ionic conductivities and phase transitions will be discussed.

References
1) A.J.E. Rettie et al., J. Am. Chem. Soc., 2018, 140, 9193−9202.
2) A.J.E. Rettie et al., Nat. Mater., 2021 (accepted), ChemRxiv preprint.

I will present our recent studies on further increasing the accessible electronic structures of the versatile family of halide perovskites by synthesizing mixed-metal compositions. Our work on halide double perovskites has prompted us to explore still more complex perovskites. In particular, I will discuss unusual new architectures that combine sublattices of different dimensionality and the consequences of incorporating three distinct stoichiometric metals in the octahedral sites.

We present atomistic modelling of uranium oxycarbide (UCO), a candidate fuel kernel for TRISO particles in the high temperature gas reactors (HTGRs). The use of UCO aims at mitigating the formation and release of CO gas [1] which is associated with the TRISO particle failure. Under operation, UCO microstructure evolves to form the high atomic mass UC(O) phase with rocksalt structure and the low atomic mass UO2(C) phase with fluorite structure [2, 3]. This study sheds light on phase stability, structural and thermodynamic quantities of both UCO phases. Defects stability, atomic self-diffusion and fission products incorporation are also investigated in the UO2(C) phase. Our results demonstrate a strong dependence on carbon content, which may have a significant impact on the fuel chemistry under operation, thereby playing an important role in the fuel performance properties such as fission products retention and release in UCO fuel kernel of TRISO particles.

[1] J. W. McMurray, T. B. Lindemer, N. R. Brown, T. J. Reif, R. N. Morris, J. D. Hunn, Ann. Nucl. Energy 104 (2017) 237 – 242.
[2] I. J. Van Rooyen, Z. Fu, Y. Yang, T. Holesinger, M. Bachhav, in: Proceedings of HTR 2018, Warsaw, Poland.
[3] J. A. Phillips, S. G. Nagley, E. L. Shaber, Nucl. Eng. Des. 251 (2012) 261 –281

Developing bi-functional electrocatalysts for both oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is crucial for enhancing the energy transfer efficiency of metal-air batteries and fuel cells, as well as producing hydrogen with high purity. Herein, a series of Ni-Co alloyed nanoparticles encapsulated in or decorated on activated charcoal and porous carbon support (BAX) synthesized using electrodeposition and employed as a bifunctional electrocatalyst for both ORR and HER. The scanning electron microscopy measurements showed optimal loading of Ni-Co nanoparticles, with a size of approximately 5–10 nm, which were uniformly dispersed on the nanoporous carbons support, with occasional aggregated of nanoparticles. The as-prepared samples exhibited effective ORR activity in alkaline media and excellent HER activity in both alkaline and acid solutions. While the cobalt nanoparticles showed an overpotential of 54 mV dec^-1, Ni–Co_3:1 showed higher current density and higher overpotential of 130 mV dec^-1 at 10 mA cm⁻² in 1 M KOH, respectively suggesting Volmer-Tafel and Volmer-Herovsky mechanism, respectively, for HER. Overall, Ni: Ni:Co_3:1/BAX on graphite rod samples displayed the best activity and stability among the series, which is comparable and superior to that of commercial 10% Pd/C. The strategy for encapsulating bimetallic alloys within porous carbon materials is promising for fabricating sustainable energy toward electrocatalysts with multiple electrocatalytic activities for alternative energy-related applications.

Complex crystal structures with subtle atomic scale details are now routinely solved using complementary tools such as X-ray and/or neutron scattering combined with electron diffraction and imaging. Identifying unambiguous atomic models for oxyfluorides, needed for materials design and structure–property control, is often still a considerable challenge. In this work, NMR crystallography and single-crystal X-ray diffraction are combined for the complete structure solution of three new compounds featuring a rare triangular early transition metal oxyfluoride cluster [Mo₃O₄F₉]^5–. After framework identification by single-crystal X-ray diffraction, 1D and 2D solid-state ¹⁹F NMR spectroscopy supported by ab initio calculations are used to solve the structures of (1) K₅[Mo₃O₄F₉]●3H₂O, (2) K₅[Mo₃O₄F₉]●2H₂O, and (3) K₁₆[Mo₃O₄F₉]₂[TiF₆]₃●2H₂O and to assign the nine distinct fluorine sites in the oxyfluoride clusters. Furthermore, ¹⁹F NMR identifies selective fluorine dynamics in K₁₆[Mo₃O₄F₉]₂[TiF₆]₃●2H₂O. These dual scattering and spectroscopy methods are used to demonstrate the generality and sensitivity of ¹⁹F shielding to small changes in bond length—on the order of 0.01 Å or less—even in the presence of hydrogen-bonding, metal–metal bonding, and electrostatic interactions. Starting from the structure models, the nature of chemical bonding in the molybdates is explained from molecular orbital theory and electronic structure calculations. The average Mo–Mo distance of 2.505 Å and diamagnetism in (1), (2) and (3) are attributed to a metal–metal bond order of unity along with a 1a²1e⁴ electronic ground state configuration for the [Mo₃O₄F₉] cluster, leading to a rare trimeric spin singlet involving d² Mo⁴⁺ ions. The approach to structure solution and bonding analysis is a powerful strategy to understand the structures and chemical properties of complex fluorides and oxyfluorides.

Growing interest in the design of functional materials with increasingly complex crystal structures calls for a more detailed understanding of structure-property relationships in inorganic solids. Whereby functional material properties are often linked to irregular bond distances, deciphering the causal mechanisms underlying bond-length variation, and the extent to which bond lengths vary in solids, has important implications in the design of new materials and the optimization of their functional properties.

Investigation of the relation between bond-length variation and the expression of functional material properties begins with systematization of chemical-bonding behavior via large-scale bond-length dispersion analysis. Completion of the largest bond-length dispersion analysis to date for inorganic solids (177,446 bond lengths hand-picked from 9210 crystal-structure refinements for oxides; 6,770 bond lengths from 720 crystal-structure refinements for nitrides ; 33,626 bond lengths from 1832 crystal-structure refinements for chalcogenides) recently enabled straightforward identification of anomalous (i.e. irregular) bonding behavior for all ions of the periodic table observed bonding to O^2-, N^3-, and S^2-/Se^2-/Te^2-. In addition to comprehensive description of bond-length variations in inorganic solids, the large amount of data on anomalous coordination environments provided by this undertaking allows (1) conclusive resolution of the causal mechanisms underlying bond-length variation in inorganic solids, and (2) quantification of the extent to which these causal mechanisms result in bond-length variation.

In a sample of 266 highly irregular coordination polyhedra covering 85 transition-metal ion configurations bonded to O^2-, the most common cause of bond-length variation is observed to be non-local bond-topological asymmetry — a widely overlooked phenomenon whose associated bond-length variation results from asymmetric patterns of a priori bond valences — followed closely by the pseudo Jahn-Teller effect (PJTE). Two new indices, Δ_topol and Δ_cryst, calculated on the basis of crystallographic site, are proposed to quantify bond-length variation arising from bond-topological and crystallographic mechanisms in extended solids; Δ_topol is defined as the mean weighted deviation between the bond valences of a given polyhedron and that of its regular variant with equal bond lengths, while Δ_cryst similarly quantifies the difference between a priori and observed bond valences. Bond-topological mechanisms of bond-length variation are (1) non-local bond-topological asymmetry and (2) multiple-bond formation, while crystallographic mechanisms are (3) electronic effects (e.g. vibronic mixing, lone-pair stereoactivity), and (4) crystal-structure effects (e.g. structural strain).

Comprehensive bond-length dispersion analyses for inorganic nitrides and chalcogenides reveal several “phenomenological gaps” compared to their oxide counterparts, thus providing synthetic opportunities via the transposing of anomalously bonded coordination units bearing functional properties into new compositional and/or structural spaces. Resolving the contribution of (static) bond-topological vs (tunable) crystallographic mechanisms of bond-length variation via the Δ_topol and Δ_cryst indices, combined with their spatial resolution within the coordination polyhedron and unit cell, is proposed to quantify the effective tunable extent of a functional property for a given crystal structure, e.g. via alteration of the responsible coordination unit(s). The known extent for which bond-topological and crystallographic mechanisms materialize into bond-length variations, provided by large-scale bond-length dispersion analyses, guides optimization of these properties within the constraints of physically realistic crystal structures. Such information is essential to the design of new materials with (1) increasingly complex crystal structures, and (2) superior functional properties.

Lignin is an alternative source of petroleum production, which consists of useful aromatic functional groups in lignocellulosic biomass. The aromatic-rich fraction in lignin is converted to biofuel through fermentation. Past studies have focused on lignin valorization under harsh conditions such as high temperature, pressure and/or additional oxidants. However, our research group has developed the dye-sensitized photoelectrochemical cells (DPECs) system for visible light induced lignin degradation at room temperature. Here, we introduce DSPECs incorporated with gold nanorods (Au NRs) for the application of lignin depolymerization. The Au NRs can be a promising candidate to increase the performance of the photocatalytic system and enhance light-harvesting efficiency. The Au NRs are immobilized to TiO₂ electrodes by using bifunctional L-glutathione (GSH) featuring dicarboxylic acid and thiol moieties. These moieties allow to anchor Au NRs to the surface of TiO₂. The TiO₂-based photoanode incorporates a surface-bound Au NRs-based photocatalyst and solution-dissolved nitroxyl mediators (e.g., 4-acetamido 2,2,6,6-tetramethylpiperidine-1-oxy) to perform solar-driven photocatalytic oxidation of the lignin model compounds under aerobic conditions with simulated solar illumination.

Metal oxide nanoparticles have a number of important technological applications in electronic devices. In particular, titanium(IV) oxide nanoparticles have potential applications in lithium ion batteries. A particular synthesis challenge with titanium(IV) oxide nanoparticles is that it has multiple possible structural phases. Differentiating between the various phases is particularly challenging when they are nanoparticles and it is crucial to be able to identify the structural phases of synthesized nanoparticles. Beyond knowing the phase distribution in the sample, it is also important to study crystallite sizes and shapes, as well as the presence of common defects, since these factors have an important impact on the functionalities. X-ray atomic pair distribution function (PDF) analysis is an important and powerful tool to use for this purpose because it reveals the local structure on nanometer length-scales, revealing quantitative information about the structural phase, particle morphology and defects in nanoparticulate materials, which is beyond the capability of traditional x-ray diffraction methods when the nanoparticles are small (< 10nm in diameter). One of the common methods for studying nanoparticles using PDF analysis is to use attenuated crystal model (ACM) to test candidate structures and extract quantitative information about them. Given the growing importance of the PDF approach in nano-materials science and chemistry, we feel that it is helpful to the community to discuss the approach in a pedagogical way. Here we address a gap in the literature and present a detailed workflow for applying ACM in a robust and accurate way, using titanium(IV) oxide nanoparticles as an example. Using this approach we find that the nanoparticles contain the bronze phase as the major phase and the anatase phase as the minor phase, obtain the structural parameters of both phases, and that the particle sizes of the major phase are probably following a lognormal distribution. This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DOE-BES) under contract No. DE-SC0012704.

Ultra high temperature ceramics, or uthc have various applications, especially in aircrafts and spacecrafts. They must have outstanding refractory properties at elevated temperatures. However a key limitation is in the toughness of these materials, which severely limits its applications. This is why they are used to make rocket engine nozzles and thermal barrier coatings. The most common example of a uhtc is silicon carbide, SiC. SiC is an important uhtc since it has a high melting temperature, high strength and is very lightweight. Yet it is still not perfect since it has low toughness like uhtcs. On the other hand many transition metal carbides have very low melting temperatures, but they are extremely tough. One of the key differences between transition metal carbides and SiC is the hybridization of the molecule. Using a combination of density function simulations and analytical analysis we examine the toughness, strength,Young’s modulus of SiC with diamond-crystal structure and HfC, ZrC, TaC, TiC, VC, and NbC with rock-salt crystal structures under symmetry-preserving and symmetry-breaking deformations. In this presentation we will discuss how certain electronic structures and hybridizations create materials with variable toughness, young’s modulus, and strength. By examining the patterns in electron redistribution we will see how the type of orbital (p,d,s) and hybridization (sp3, spd) affect the mechanical properties. By examining how mechanical properties are developed due to the electronic structure we can find alternatives or supplements to SiC. This new understanding will help us create better materials for nozzles of rockets and other elements of rockets.

Current Li-ion batteries store charge by intercalation and deintercalation of Li⁺ into a host material such as a transition metal oxide. Capacities are typically limited to less than one equivalent of Li⁺ per transition metal and the charge compensation is largely localized on the transition metal. Utilizing the electronic states of the anions represents an interesting way to increase the Li⁺ that can be stored to reach so-called "Li-rich" regimes in which greater than one equivalent of Li⁺ is stored per transition metal. We will discuss a family of Li-rich sulfides that undergo reversible multi-electron redox involving both the transition metal (M) and the sulfide. Anion subsitution allows us to tune the covalency of the M-anion bond, systematically control the voltage of anion oxidation, and control the distribution of eletroactive atoms in the material during the charge. Anion oxidation causes interesting strutural changes that will be discussed, along with the structure-property relationships that control anion oxidation reversibility.

Strongly correlated transition metal oxides are renowned for their ability to phase-segregate into multiple competing states with distinct electronic properties. Prominent examples include colossal magnetoresistant (CMR) manganites, where chemical disorder mediates segregation into ferromagnetic metallic and antiferromagnetic charge/orbital-ordered states¹. This phenomenon has drawn considerable attention in oxides containing 3d transition metals, yet reports are comparatively scarce for their 4d/5d counterparts.

In recent years, 6H-perovskites of the form Ba₃B’M₂O₉ (B’ = rare earth, M = 4d/5d transition metal) have been investigated for their potential to exhibit spin-liquid ground states². This structure type is distinguished by bioctahedral M₂O₉ dimers; short distances between the M cations, combined with spatially diffuse 4d/5d orbitals, can facilitate the formation of quasi-molecular M₂O₉ clusters with spin-gapped excitations³. However, most Ba₃B’M₂O₉ compositions feature magnetic moments localised on the M cations so that the formation of quasi-molecular clusters is suppressed.

We have observed an unprecedented electronic phase segregation (EPS) in the new 6H-perovskite Ba₃SrMo₂O₉. This is the first report of a Ba₃B’M₂O₉ composition where M = Mo. Below 230 K, Ba₃SrMo₂O₉ segregates into two electronically distinct phases: a hexagonal P6₃/m phase consisting of weakly interacting Mo⁵⁺₂O₉ spin dimers, and a monoclinic P2₁/m phase containing a 50:50 mixture of quasi-molecular Mo⁵⁺₂O₉ clusters and localised Mo⁵⁺ spins. We propose this segregation has a unique origin related to competition between direct Mo–Mo bonding and Mo–O–Mo superexchange. In contrast to established electronic materials like CMR manganites, EPS appears to manifest without the aid of charge order or chemical disorder. Accordingly, Ba₃SrMo₂O₉ comprises a seemingly unique electronic material.

References

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In this study, we report on the investigation of the electronic and chemical structure of supported GaRh alloys as model systems for the active phase in Supported Catalytically Active Liquid Metal Solutions (SCALMS). SCALMS is a novel class of single atom catalysts comprised of a liquid metal matrix and a diluted catalytically active transition metal. We prepared a series of gallium-rhodium samples with different Ga/Rh molar ratios and tracked the evolution of the surface electronic structure decreasing Rh concentration (from 100 % to 1 %). We employed Photoemission Spectroscopy (PES) to probe the shallow core levels of the top layers of the GaRh alloys. Our results reveal a narrowing of the rhodium 4d-band and the formation of Ga-Rh intermetallic compounds (IMCs). Furthermore, we discuss annealing and surface oxidation effects. We observe the dissolution of GaRh IMCs during annealing and the incorporation of Rh into the formed gallium oxide (GaO_x) layers as a result of surface oxidation. These results contribute to a more fundamental understanding of the valence band structure of GaRh SCALMS and its relation to the catalytic reactivity.

Our group investigates new materials synthesized by the anionic four-coordinate/penta-coordinate molecules with cyanides as building units, combined with various metal ions or organic cations. They often exhibit interesting dynamic structural characteristics such as thermal expansion and melting. Recently, we synthesized a cyanide-bridged three-dimensional framework FePd(CN)₄ incorporating four-coordinate iron(II) ions. FePd(CN)₄ was synthesized by a thermal treatment to remove waters from a two-dimensional six-coordinate iron(II) coordination polymer Fe(H₂O)₂Pd(CN)₄.4H₂O. FePd(CN)₄ consists of a two-fold interpenetrated PtS topology three-dimensional structure where four-coordinate iron(II) ions has intermediate geometry between tetrahedral and square planar geometries. FePd(CN)₄ exhibits large anisotropic thermal expansion behavior including uniaxial colossal TE and negative 2D-TE. Moreover, we are also focusing on the penta-coordinate mixed-anion units to construct polar skeletons. Such compounds achieved a characteristic polarity switching via anion manipulation along polar axis in crystals.

Photo-induced water splitting using semiconductor photocatalysts has attracted considerable attention for producing H2 as a clean energy carrier, while the effective utilization of visible light is imperative to achieve the desired efficiency for practical applications.[1] Various mixed-anion compounds such as oxynitrides, oxysulfides, and oxyhalides have been extensively studied as promising photocatalysts for visible light-induced water splitting, because their valence band maximum (VBM) values are generally more negative than those of conventional oxides, due to the significant contribution of high-energy p orbitals of the non-oxide anions (e.g., N-2p, S-3p, Br-4p, and I-5p) mixed with O-2p. Some tantalum oxynitrides (TaON and BaTaO2N) have been successfully employed as a H2-evolving photocatalyst in Z-scheme water splitting systems, combined with another O2-evolving photocatalyst such as WO3.[2] However, most of mixed-anion compounds suffer from facile self-oxidative deactivation of non-oxide anions by photogenerated holes, thereby imposing surface modifications such as loading some cocatalysts to circumvent the oxidative deactivation. For example, we have demonstrated that the loading of IrO2 or CoOx nanoparticles as a cocatalyst for water oxidation on such oxynitrides suppress the self-oxidative deactivation to some extent, and thus enable us to employ such surface modified (oxy)nitrides (TaON and Ta3N5) as O2-evolving photocatalysts in Z-scheme systems with (IO3–/I–) redox.[2] Such surface modification has been proven effective for the fabrication of (oxy)nitride-based photoanode. The porous tantalum oxynitrides (TaON or BaTaO2N) electrode prepared on conducting substrates showed relatively stable O2 evolution with significantly high quantum efficiency in an aqueous solution, after loading of effective cocatalyst nanoparticles for water oxidation.[3,4]

We have recently demonstrated that Sillén–Aurivillius type perovskite oxyhalides such as Bi4NbO8Cl can stably and efficiently oxidize water to O2 under visible light without any surface modifications, and also exhibits a stable Z-scheme water splitting[5] when coupled with a H2-evolving photocatalyst.[4] It was revealed that the VBMs of these materials consist mainly of O-2p orbitals, instead of Cl-3p (or Br-4p), but their positions are much more negative than those of conventional oxides. Thus, they possess narrow bandgaps for visible light absorption as well as sufficiently negative CBMs for water reduction. DFT calculation visualized a fairly strong hybridization between the Bi-6s and O-2p orbitals, which can explain why the O-2p orbitals are elevated in energy, combined with the result on Madelung site potential analysis that can rationalize the origin of high energy of O-2p orbital in these materials.[6, 7] Since O– anions are known to be relatively stable, photogenerated holes populated at the O-2p orbitals will not lead to self-decomposition but to oxidize water. These results could provide new strategies for developing durable materials for water splitting under visible light, by manipulating the interaction between post-transition metal s orbitals and O-2p orbitals.

[1] R. Abe, J. Photochem. Photobiol. C: Photochem. Rev. 2011, 11, 179.
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[7] H. Kunioku, H. Kageyama, R. Abe et al., J. Mater. Chem. A 2018, 6, 3100.

Metal fullerides are molecule-based strongly correlated electron systems, whose properties can be tuned by both changes in bandfilling and bandwidth (or equivalently interfullerene separation). The electronic properties of trivalent cubic fullerides, A₃C₆₀ (A = alkali metal) vary with lattice volume, resulting in a dome-shaped superconducting transition temperature (T_c) dependence adjacent to an antiferromagnetic (AFM) insulating phase - fingerprint of strong electron correlations. As evidenced by the maximum T_c ~ 38 K (highest among molecular superconductors) and upper critical magnetic field H_c2 ~ 90 T (highest among isotropic superconductors), fullerene is a unique materials platform in condensed matter physics. The flexible volume tunability of fullerides by chemical/physical means has inspired the search for possible emergence of superconductivity at other bandfillings. Indeed, there are superconducting fullerides in octavalent (Ba₄C₆₀, T_c ~ 7.7 K) and nonavalent (A₃Ba₃C₆₀, T_c ≤ 6 K) materials in which the Fermi level lies in the t_1g (LUMO+1)-derived band.
The properties of pentavalent fullerides M₂AC₆₀ (M = divalent cation) have not been explored systematically although some theories do suggest the emergence of superconductivity. Two decades ago, studies showed that non-superconducting Ba₂AC₆₀ (A = K, Rb, Cs) [1, 2] form a cation-ordered face-centered-cubic (fcc) structure, in which Ba²⁺ and A⁺ ions occupy small tetrahedral and large octahedral holes, respectively. In particular, there was little change in the lattice volume with changing the A⁺ ion (= K, Rb, Cs) size in Ba₂AC₆₀ because the octahedral holes are of a much larger size (~ 2.06 Å) than that of the A⁺ ions (e.g. Cs⁺ ~ 1.67 Å). Therefore, substituting the tightly-packed Ba²⁺ ion in the small tetrahedral hole (~ 1.14 Å) with other divalent cations may allow access to unexplored regions in the pentavalent fulleride phase field.
Here we present our recent work on the families of ternary fullerides, M₂AC₆₀ (M = alkaline-earth or rare-earth metal). We have successfully prepared cation-ordered M₂AC₆₀ phases via solid-state synthesis, systematically varying both M²⁺ and A⁺ ions. Interestingly, X-ray diffraction profiles show that smaller M²⁺ ions lead to non-cubic crystal structures (orthorhombic) in which the M²⁺ ions are shifted to off-centered positions, in contrast to isoelectronic fcc-Ba₂AC₆₀. Both the reduction in symmetry and the pentavalent character of the fulleride units are confirmed by Raman spectroscopy. The electronic properties of the materials have been studied by SQUID magnetometry, and EPR and NMR spectroscopy. Complementary synchrotron X-ray absorprion studies reveal intriguing mixed valence behavior in the rare-earth fulleride systems.

[1] A. C. Duggan et al., Chem. Commun. 10, 1191-1192 (1996)
[2] T. Yildirim et al., Phys. Rev. B 54, 11981-11984 (1996)

Since the functionalities of materials usually originate from the spatial inhomogeneity in the associated systems, the local inhomogeneities such as defect, surface, interface, and nanostructure often play an important role for developing sophisticated materials. In this paper, among the many phenomena related to inhomogeneity in materials, we will report our computational research on the modulation of electronic structure by the solute elements for the long period stacking ordered (LPSO) phases.
In 2001, Kawamura et al. have developed a novel Mg₉₇Zn₁Y₂ (at. %) alloy using rapidly solidified powder metallurgy processing [1]. It has extremely high tensile yield strength of 610 MPa and elongation of 5% mechanical strength at room temperature. After that, other high-strength Mg alloys have been reported one after another [2-5] which alloys are usually composed of Mg, transition metal (TM) elements, and rare earth (RE) ones. It has also been reported that the reinforced phases in these Mg alloys contain stacking faults (SF) with long periodicity along the c-axis, and solute elements of TM and RE segregate into these SF regions [1-5]. Since the structural modulation and the chemical modulation are experimentally confirmed to be synchronized in the reinforced phases, the associated phases in these Mg alloys are called the synchronized long period stacking ordered (LPSO) phases.
Some investigations on the thermodynamic stability for LPSO phases have also been reported so far [6-8]. Egami et al. have recently examined the thermodynamic behaviors of dilute Mg-Zn-Y ternary alloys to form a unique solute-enriched stacking-fault based on the calculation of phase diagrams (CALPHAD) method along with the first-principles analyses [8]. They have found that the co-segregations of Zn and Y solute elements at the stacking faults make the fault layers more stable than the hcp-Mg matrix-base configuration. Furthermore, within the stacking faults, spinodal-like decomposition into the Mg-rich solid-solution and the Zn/Y-rich L12-type order phase is also reported to cause a significant reduction of the total Gibbs energy of the system. These results based on the CALPHAD method stimulate our curiosity as to how alloying elements associated bring about the changes of atomistic interactions in the LPSO structures.
We have presented the theoretical studies on the energetics of metallic polytypes including the LPSO structures from the viewpoint of interlayer interactions [9, 10]. Our theoretical analyses have suggested that the relatively long-distance interatomic interaction is a significant factor for the energetic stabilization of the LPSO composed of the close-packed structures [10]. Since the phase stability is generally determined by the electronic states, the associated electronic structure can provide a more basic knowledge on the thermodynamic phase stability for the LPSO phases. In the present work, comparative studies are carried out on the electronic structure of Mg₉₂Zn₁₂Y₁₆, Mg₁₀₈Zn₁₂, Mg₁₀₄Y₁₆, and Mg₁₂₀ with two and/or ten stacking layers based on first-principles calculations within the density functional theory. In our presentation, we will discuss the correlation between the modulation of electronic structure by Zn and Y solute elements and the phase stability of the LPSO-Mg structures.

[1] Y. Kawamura et al., Mater. Trans. 42 (2001) 1172-1176.
[2] S. Yoshimoto et al., Mater. Trans. 47 (2006) 959-965.
[3] Y. Kawamura et al., Scr. Mater. 55 (2006) 453-456.
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[5] S. Izumi et al., Mater. Sci. Forum 654-656 (2010) 767-770.
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[7] S. Iikubo et al., Phys. Rev. B 86 (2012) 054105
[8] M. Egami et al., Mater. Des. 188 (2020) 108452.
[9] K. Moriguchi et al., MRS Advances 6, 163-169 (2021).
[10] S. Ogane and K. Moriguchi, MRS Advances 6, 170-175 (2021).

The Kepler conjecture in 1611 states that no arrangement of equally sized spheres filling space has a greater average density than that of the face-centered cubic and/or hexagonal close packing arrangements [1]. This conjecture has been only recently proved by the Flyspeck project team [2]. The polytypes are characterized by a stacking order with a given repeating unit along a directional axis and are theoretically possible to have endless permutations of the sequences. The close-packed (CP) polytypes in which the associated stacking is composed of the CP planes are, therefore, considered to be the crystalline systems realizing the Kepler conjecture. Significant changes in physical properties due to the intentional polytype formation have also been reported in various fields of metallic and semiconductive systems [3-5].
Predicting polytype phase stability for a material has still been a long-standing issue in condensed matter physics and/or materials science [6, 7]. This situation stems from the fact that the atomistic interactions on polytype energetics might be quite complex and delicate despite the simplicity of their geometrical structure. We have recently presented a theoretical consideration on the CP polytype total energetics using the geometrical analysis for the correlation between interlayer interactions and interatomic ones [8]. These results suggest that short-range interactions are not enough to describe the CP polytype energetics and provide significant insights for creating interatomic models successfully showing the polytypes other than 3C and 2H structures as a ground state, those have never yet been implemented as far as the authors know.
Since the ground state for metallic lanthanum (La) is the double hexagonal CP structure (DHCP), the La can be an important reference system for studying polytype formation mechanism. It is, therefore, possible to investigate the essential factors determining the polytype selection rule through the dynamical analyses for metallic La. In the present work, we report on the process of constructing an interatomic potential for metallic La, that can be used for molecular dynamics (MD) simulations. The Embedded Atom Method (EAM) type functions reported by Mishin et al. [9] are adopted for constructing the numerical potential model in this study. Since our previous theoretical studies have shown that the long-range nature of interactions is an important facter for determining the ground state of polytype [7, 8], the cutoff radius has been carefully selected considering the computational cost.
Our interatomic potential can well describe the total energy vs. volume curves for various crystal structures and the Bain path energetics along the fcc-bcc structures deduced by the first-principles calculations. This potential is, therefore, an interaction model for metallic polytypes that can describe not only the energetics but also the lattice dynamics such as lattice dynamical instability, which is generally significant for the phase transition phenomena [10, 11]. Our research is at the stage of working on MD analyses for the polytype formation mechanism using this potential. In the presentation, other basic properties, transferability of the potential constructed, and some results of the MD analyses will be also discussed.

[1] J. Kepler, The Six-Cornered Snowflake, 1966 translation by C. Hardie (Clarendon Press, Oxford, 1611), ISBN: 0198712499.
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[8] S. Ogane and K. Moriguchi, MRS Advances 6, 170 (2021).
[9] Y. Mishin et al., Phys. Rev. B 59, 3393(1999).
[10] K Moriguchi and M Igarashi, Phys. Rev. B 74, 024111 (2006).
[11] G. Grimvall et al., Rev. Mod. Phys. 84, 945 (2012).

Precise identification of the distortion, tilt, and rotation of octahedra is an essential step toward understanding the structure-property correlation in view of the fact that the physical properties of ABX₃ perovskite-based oxides strongly depend on the geometry of oxygen octahedra containing B site cations. In this work, we find out an crucial electrostatic origin responsible for Jahn-Teller-type tetragonal elongation of oxygen octahedra during atomic-level direct observation of two-dimensional [AX] interleaved shear faults in five different perovskite-type materials. When the [AX] sublayer has a net charge, for instance [LaO]⁺ in LaNiO3, significant tetragonal elongation of oxygen octahedra at the fault plane is observed and it blocks the strong repulsion between the consecutive [LaO]⁺ layers. Moreover, our findings on the distortion induced by local charge are identified to be a striking common feature in A_n+1B_nX_3n+1-type Ruddlesden-Popper structure with charged [AX] sublayers. This study thus demonstrates that electrostatic origin local charge is a crucial factor significantly affecting the crystal structure of complex oxides.

Ferroelectric materials exhibit the characteristic photovoltaic (PV) effect that can generate photovoltage far exceeding their bandgap energy (E_g)^[1]. The ferroelectric photovoltaic (FPV) effect arises from polarization-driven carrier separation under light irradiation and the absence of limitation in the photovoltage by the bandgap is a great advantage of the FPV effect over the conventional PV effect in p-n junctions of semiconductors. Since typical ferroelectric oxides have wide bandgap, a vanishingly small photoresponse under visible light has been an issue to be overcome. Though bandgap tuning can partly improve the photoresponse, narrowing of the bandgap is, in principle, accompanied by a substantial loss of ferroelectric polarization. In this study, to enhance the visible-light activity of the ferroelectric materials without sacrificing the ferroelectric polarization, we propose an approach, ‘gap-state’ engineering, in which defect states within the bandgap act as a stepping stone for photocarrier generation^[2,3]. We investigate the FPV effect in ferroelectric BiFeO₃ (BFO) epitaxial thin films doped with various transition metals (TMs). Impact of the TM doping on electronic structures and the PV response were investigated by combined analysis using the density functional theory (DFT) calculations and experimental measurements for the TM-doped BFO epitaxial thin films.
The DFT calculations revealed that substitution of Mn³⁺ for Fe³⁺ generates doubly degenerated half occupied defect level derived from Mn-3d orbitals within the bandgap of BFO. The defect level i.e., gap states, locates at 0.8 eV above the valence band maximum, which corresponds to 1.9 eV below the conduction band minimum. Experimental analyses of the PV properties were performed for epitaxial thin films of undoped BFO and 5% Mn-doped BFO (BFMO) with a single domain structure prepared by the pulsed-laser deposition (PLD). We found that both the photocurrent density and the photovoltage were markedly enhanced by the introduction of small amount of Mn especially under illumination with photon energy lower than E_g.
These results present that the gap states formed within the bandgap enables two-photon excitation and provides the enhanced PV response over a broad spectral range. This approach utilizing the gap states will be promising route to the development of visible-light-active ferroelectrics without sacrificing the ferroelectric polarization.
[1] A. M. Glass et al., Appl. Phys. Lett., 25, 233 (1974).
[2] H. Matsuo et al., Nat. Commun., 8, 207 (2017).
[3] Y. Noguchi et al., Nat. Commun., 11, 966 (2020).

Melting is a common phenomenon in our daily life and it is fundamental to daily life, basic science and technology. Therefore, understanding and controlling the melting phenomenon becomes important in various disciplines. In particular, pre-melting which occurs below the melting temperature has been studied for in-depth understanding of melting and numerous studies have shown that pre-melting usually occurs in 2-dimensional defects like surface, grain boundaries and dislocations. In this study, using atomic-scale direct observation of scanning transmission electron microscopy (STEM) and acceptor doepd BaCeO₃ as model system, pre-melting phenomena also can occur in Ruddlesden Popper(RP) faults in ABO₃ type perovskite oxide. Chemical composition analysis and STEM images demonstrate that the amorphous phase is the RP-mediated liquid phase and as sintering temperature goes up, the thickness of amorphous phase in RP faults getting thick. And with electron beam irradiation, re-crystallization of the amorphous layer can occur. To the best of our knowledge, the present study demonstrates for the first time ever observation of pre-melting in RP faults providing a broader academic depth to pre-melting or melting.

The discovery of new families of two-dimensional (2D) van der Waals (vdW) layered materials has always attracted great attention to pursue beyond graphene. It has been challenging to artificially develop the van der Waals bonded layer structure that is constructed by the stacking of honeycomb atomic lattice composed of two elements as in hexagonal boron nitride. In this talk, a new class of 2D vdW materials, layered Zintl phases will be introduced. These new 2D layered Zintl phases can allow unlimited extent of 2D science in terms of the diversity of materials and their physical properties. A new class of 2D materials was developed from a 3D structured material that has (1) a multicomponent system, (2) primary atomic bonds in three-dimensionality, (3) thermodynamic and chemical stability, and (4) diversity in chemical compositions. Through the dimensional manipulation of crystal structure, we create an unprecedented 2D vdW zinc antimonide (2D-ZnSb), which is the layered Zintl phase with sp²-hybridized bonding characters in Zn-Sb honeycomb atomic layers [1]. The vdw layered structure of 2D-ZnSb is evolved by selectively etching the lithium cations from the layered LiZnSb Zintl phase that is formulated by alloying the lithium atoms into sp³-hybridized bonded three-dimensional ZnSb (3D-ZnSb), demonstrating the bidimensional polymorphism of 3D- and 2D-ZnSb. Finally, the recent experimental and theoretical studies on diverse physical properties obtained in the new 2D materials will be discussed with the focus on the three-dimensional topological Dirac semimetal of KZnBi compound [2].

[1] J. S. Song et al., Sci. Adv. 2019;5 : eaax0390.
[2] J.S. Song et al., Phys. Rev. X, online published, https://journals.aps.org/prx/accepted/7b077K0cI2f1550f853344b9421091d5157265e34

Layered transition metal compounds have been extensively investigated due to their unconventional electronic/magnetic properties. Topochemical reactions, which introduce/withdraw ions into/from host materials retaining their lamellar structures, serve as one of the most effective tools to design such low-dimensional materials. Despite their huge advances during the recent decade, most of topochemical reactions rely on redox activities of transition metal cations in host lattices so that their charge balance are respected throughout the process. Conversely the redox activity of molecular anions for topochemical syntheses remains almost unexplored. This study demonstrates the topochemical reaction of layered polychalcogenides or oxychalcogenides containing pre-formed (Sn)2- (n=2,3) oligomers with transition metals or alkali metals to form (metastable) layered compounds at low temperature [1,2,3]. Structure prediction calculations and advanced electron microscopy techniques confirms that depending on the metal nature the reaction with the oligomers lead either to the intercalation of the metallic species [1,2], or to the deintercalation of part of the chalcogen atoms [3]. This study paves therefore the way to the design of (oxy-)chalcogenide compounds with interesting potential applications.
[1] S. Sasaki, D. Driss, E. Grange, J.-Y. Mevellec, M. T. Caldes, C. G.-Deudon, S. Cadars, B. Corraze, E. Janod, S. Jobic, L. Cario, Angew. Chem. Int. Ed., 57, 13618-13623 (2018).
[2] S. Sasaki, M. Lesault, E. Grange, E. Janod, B. Corraze, S. Cadars, M. Teresa Caldes, C. Guillot-Deudon, S. Jobic, L. Cario, Chemical Communications 55, 6189 (2019).
[3] S. Sasaki, M. T. Caldes, C. Guillot-Deudon, I. Braems, G. Steciuk, L. Palatinus, E. Gautron, G. Frapper, E. Janod, B. Corraze, S. Jobic, L. Cario, Nat. Commun. 12, 3605 (2021).

Several ways can be used to achieve magnetoelectric (ME) effects [1 and references herein] such as strain-mediated coupling between ferroelectric and (anti)ferromagnetic composites, compounds hosting both d⁰ ferroelectricity and 4f or dⁿ magnetism and lone-pair cation (such as BiFeO₃ [2]). In general, for the latter, the electric polarization is large but the ME coupling weak. Another class of multiferroics (MF), also called spin-induced MF, gathers the 3dⁿ magnetic compounds whose magnetic symmetry breaks both space-inversion and time-reversal. These type-II MFs are very often compounds exhibiting large ME coupling. Searching for large ME coupling, the linear ME (LME) compounds are also attracting as a third class of ME. Though these centrosymmetric compounds are not MF, a magnetic field application induces a polarization, with P proportional to H, as the prototype α-Cr₂O₃ corundum [3,4].
For solid state chemists, this structural type forms a very broad field to play with, as illustrated for instance by the numerous isomorphic transition metal niobates and tantalates of composition M₄A₂O₉ (429), with M a divalent cation (M = Co, Fe, Mn) [5]. Recent re-investigations of Co₄Nb₂O₉ magnetic structure [6-8], a LME exhibiting one of the largest ME coefficient (≈ 30 ps/m [6]), triggered a surge of interest for these M₄A₂O₉ crystallizing in a trigonal structure (SG: P-3c1) and which magnetic structure is still a matter of debate [5-8].
Among the 429, solely cobalt and manganese niobates and tantalates were known to be LME till 2018 [9-12], after which the Fe₄Nb₂O₉ [13] and Fe₄Ta₂O₉ [14-15] ME were reported. From the synthesis side, these oxides containing divalent iron are difficult to prepare as pure phases.
Interestingly, there exist M_4-xM’_xA₂O₉ solubility ranges, involving a careful control of the (PO₂,T) synthesis conditions depending on M and M’ and on x, which open the route to produce new LME and ME behaviours. In particular, to exacerbate the P response to H, the M/M’ magnetic cations disorder can be exploited to modify the magnetization H dependence. The preliminary results obtained by mixing divalent Fe and Co have allowed us to produce a much steeper P(H) dependence, the Fe₂Co₂Nb₂O₉ compound exhibiting ME properties overpassing those of the Co₄Nb₂O₉ and Fe₄Nb₂O₉ end members.
On the opposite, considering that Ni₄Nb₂O₉ is orthorhombic, ferrimagnetic and not ME [16], the Ni solubility in M_4-xNi_xNb₂O₉ with M=Co or Fe has been investigated. These magnetic substitutions by Co or Ni are now compared to those by diamagnetic ones.
The synthesis, characterizations of the crystal/magnetic structures and magneto(di)electric properties of several such corundum - derived compounds, including Mn₃WO₆ [17], will be presented to illustrate the richness of these systems to generate new (L)ME candidates.


[1] N. A. Spaldin et al., Nature Materials 18, 203–212 (2019).
[2] J. Wang et al., Science 29, 1719-1722 (2003).
[3] I. E. Dzyaloshinskii, Zh. Eksp. Teor. Fiz. 37, 881 (1959).
[4] D.N. Astrov, Zh. Eksp. Teor. Fiz. 38, 984 (1960).
[5] E. F. Bertaut et al., J. Phys. Chem. Solids 21, 234 (1961).
[6] N. D. Khanh et al., Phys. Rev. B 93, 075117 (2016).
[7] N. D. Khanh et al., Phys. Rev. B 96, 094434 (2017).
[8] G. Deng et al., Phys. Rev. B 97, 085154 (2018).
[9] Y. M. Cao et al., Scientific Reports 7, 14079 (2017).
[10] N. D. Khan et al., Applied Physics Letters 114, 102905 (2019).
[11] B. B. Liu et al., Materials Letters 164, 425 (2016).
[12] N. Narayanan et al., Physical Review B 98, 134438 (2018).
[13] A. Maignan et al., Physical Review B 97, 161106 (2018).
[14] A. Maignan et al., Physical Review Materials 2, 091401 (2018).
[15] S. N. Panja et al., Physical Review B 98, 024410 (2018).
[16] E. Tailleur et al., J. of Appl. Phys. 127, 063902 (2020).
[17] A. Maignan et al., Chem. Mater. 32, 5664 (2020).

CuBi₂O₄ is an emerging p-type semiconductor for applications as a photocathode in photoelectrochemical (PEC) solar fuel production. Recently, we have examined p-CuBi₂O₄ thin films with a comprehensive spectroscopic and first principles characterization methodology to describe its fundamental electronic and optical properties while addressing intrinsic limitations in the observed PEC performance [1]. In this talk, connections will be established between electronic structure, optical properties, and PEC performance through a combination of X-ray spectroscopies (X-ray absorption spectroscopy, X-ray emission spectroscopy, and resonant inelastic X-ray scattering) and density functional theory (DFT) based modeling. The results provide the basis for the understanding of the observed polaron limitations in CuBi₂O₄. In addition, we investigate the presence and working mechanism of natural defects, through experimental and first principles approaches. Two main types of defects, oxygen vacancies and hydrogen impurities, are found in CuBi₂O₄ thin films with a high level. Our DFT results show that oxygen and hydrogen defects act as donor impurities, and therefore, their suppression increases the carrier density and enhance the performance of CuBi₂O₄. Our work aims at broadening the understanding of materials governed by polaronic limitations as well as the investigation of defects in p-type metal oxide semiconductors to further advance their application in solar-fuels generating systems.

References:
[1] Cooper et al., Chem. Mater. 33, 934 (2021).

The most commonly used cathode materials in Li-ion batteries are layered lithium metal oxides (LiTMO₂, TM = transition metal ion) due to their large theoretical capacities (~280 mAh/g).^[1] Among the layered cathode materials, lithium nickel manganese cobalt oxides (Li(Ni_xMn_yCo_1-x-y)O₂ or NMC) exhibit promising electrochemical properties, which primarily depends on the atomic composition of Ni, Mn and Co in the structure.^[2] Several NMC compositions such as NMC 111, 442, 532, 622 and 811 (the three numbers denote the ratio of Ni to Mn to Co) have been studied extensively. This study systematically studies the effect of TM ratio on the electrochemical behavior of cathode materials and why these particular TM ratios have emerged to be of highest commercial interest.
To do so, this study focuses on cathode material compositions in the series LiNi_0.5Mn_0.5-xCo_xO₂ (0 ≤ x ≤ 0.3) that includes LiNi_0.5Mn_0.3Co_0.2O₂ or NMC 532. Three cathode materials NMC 541, NMC 532 and NMC 523 were synthesized by systematically varying the concentrations of Mn and Co but keeping the Ni content constant. This systematic variation in atomic percentages of Mn and Co leads to different Ni²⁺ to Ni³⁺ ratios (4:1 for NMC 541, 3:2 for NMC 532, and 2:3 for NMC 523). For each composition, two morphological variants with different surface areas were made. The compounds were structurally and electrochemically characterized. The results of this study show that NMC 532 achieves a balance between a larger number of antisite defects with increasing Ni²⁺ (e.g. NMC 541) and the higher surface reactivity found with increasing Ni³⁺ (e.g., NMC 523), both of which lead to capacity fade. This explains why NMC 532 is the highest performing material in the LiNi_0.5Mn_0.5-xCo_xO₂ series.

References:
[1]. C. M. Julien, A. Mauger, K. Zaghib, H. Groult, Inorganics. 2 (2014) 132–154.

[2]. R. Van Noorden, Nature. 507 (2014) 26–28.

Scientists and practitioners have long dreamt of designing materials with novel properties. Yet, a hundred years after quantum mechanics lay the foundations for a systematic description of the properties of solids, it is still not possible to predict the best material in applications such as photovoltaics, superconductivity or thermoelectric energy conversion. This is a sign of the complexity of the problem, which is often exacerbated by the need to optimize conflicting material properties. Hence, one can ponder if design routes for materials can be devised.
In recent years, the focus of our work has been on designing advanced functional materials with attractive opto-electronic properties, including phase change materials, thermoelectrics, photonic switches and materials for photovoltaics. To reach this goal, one can try to establish close links between material properties and chemical bonding. However, until recently it was quite difficult to adequately quantify chemical bonds. Some developments in the last decades, such as the quantum theory of atoms in molecules [1] have provided the necessary tools to describe bonds in solids quantitatively. Using these tools, it has been possible to devise a map which separates different bonding mechanisms [2]. This map can now be employed to correlate chemical bonding with material properties [3]. Machine learning and property classification demonstrate the potential of this approach. These insights are subsequently employed to design phase change as well as thermoelectric materials [4,5]. Yet, the discoveries presented here also force us to revisit the concept of chemical bonds and bring back a history of vivid scientific disputes about ‘the nature of the chemical bond’.

Intense recent research efforts are directed toward the development of novel materials morphologies, such as nanoparticles, able to efficiently convert ionizing radiation into light. Fluorescent or scintillating materials attract great interest in lighting technology, solar applications, theranostics and self-lighting photodynamic therapy. Moreover, additive manufacturing advancements suggest the possibility to fabricate nanoparticle-based materials applicable in several fields. Therefore, novel luminescent nanomaterials free from toxic or expensive elements, such as rare earths, are desirable building blocks for promoting future developments in all these fields.
In this work, the structural and morphological properties of pure and Ti-doped HfO2 nanoparticles are studied in dependence of calcination temperatures leading to a fine tuning of either morphology and functional defect configuration. Indeed, tuneable optical features are observed in undoped monoclinic HfO2 nanocrystals and their dependence on the structural properties of the material at the nanoscale are disclosed. TEM, XRD, XPS, and surface area data combined with the analysis of the luminescence allowed us to identify the dual nature of the broad emission at 2.5 eV, where an ultrafast defect-related intrinsic luminescence overlaps with a slower emission ascribed to extrinsic Ti impurities.[1]
When titanium is employed as luminescence activator, a bright emission is observed under excitation with both UVC radiation and X-rays. [2] These results represent useful insights toward the design of novel promising nanomaterials suitable for rare-earth-free UV pumped white LEDs based on intrinsic defect engineering, and for X-ray triggered oncological therapies by using the Ti (IV)-related bright radioluminescence to excite photosensitizer molecules for singlet oxygen generation.

[1] Villa, I.; Vedda, A.; Fasoli, M.; Lorenzi, R.; Kränzlin, N.; Rechberger, F.; Ilari, G.; Primc, D.; Hattendorf, B.; Heiligtag, F. J.; Niederberger, M.; Lauria, A., Size-Dependent Luminescence in HfO2 Nanocrystals: Toward White Emission from Intrinsic Surface Defects. Chem. Mater. 2016, 28 (10), 3245-3253.

[2] Villa, I.; Moretti, F.; Fasoli, M.; Rossi, A.; Hattendorf, B.; Dujardin, C.; Niederberger, M.; Vedda, A.; Lauria, A., The Bright X-Ray Stimulated Luminescence of HfO2 Nanocrystals Activated by Ti Ions. Advanced Optical Materials 2020, 8 (1), 1901348.

Conventional LEDs consist of heterojunction structures in which the active region (usually a multi quantum well stack (MQW)) is embedded in-between the layers in a p-n junction. Recent developments have made it possible to decouple the active region from the p-n junction and demonstrate a working LED where electron and holes are injected by carrier diffusion into the MQW. [1] In order to improve this architecture further we propose to implement it in nanowire (NW) structures. From theoretical simulations it has been shown that NWs with diameter below a threshold value, trap light very poorly because the electric field is not kept inside the NW [2]. To make use of this property and to separate the active region from the p-n junction we aim to make heterostructures with branched nanowires, where core p-i-n NWs with a high bandgap are grown, followed by a subsequent growth of NW branches with lower bandgap. The difference in bandgap would induce carrier diffusion towards the branches, which have a small diameter (~25-50 nm), where the carriers can recombine, and as a consequence light is emitted. The benefits of using nano scale building blocks in this architecture is not only that lattice matching requirements are relieved, but opens up to completely avoid the challenge of total internal reflection as the light emitted has much longer wavelength than the nanowire diameter.

We have developed a cost beneficial method to deposit Au particles along the entire length of the core nanowires which act as catalyst for the growth of the branches. A stock solution of HAuCl₄3H₂O (10 mM) was prepared, in order to deposit Au particles on the sidewalls of the cores by submerging the substrates inside the solution. To study the deposition of the Au particles, core p-i-n InP NWs in a hexagonal pattern with a pitch of 1 m and a length of approximately 2 m were grown on InP(111):Zn substrates by Metal Organic Vapor Phase Epitaxy. For our study we have varied the concentration of the solution by diluting the stock solution into concentrations of 0.25, 0.5, 1, 2 and 4 mM and we have varied the temperature of the solution during deposition and the time which we submerge the substrates to find the optimal deposition parameters. To study the nucleation and growth of the branches we have grown InP branches by varying the trimethylindium (TMIn) flow and keeping growth time constant at 1 minute. By changing the concentration of the solution, we can control the growth direction of the branches to (111B) or the (110) direction.

In conclusion, we have deposited Au catalyst particles on the sidewalls of core NWs. We have studied the parameters for the deposition such as the solution concentration and temperature and the time of deposition. We have successfully grown InP branches on p-i-n InP cores and aim to develop heterostructures as the next step with InGaP p-i-n cores and InP branches leading to the intended charge carrier diffusion needed for these type of light emitting diodes with promise to high efficiency at low cost.

[1] Riuttanen, L., et al. Diffusion injected multi-quantum well light-emitting diode structure. Applied Physics Letters, 104(8), 081102, (2014). [2] Anttu, N., & Xu, H. Q. Journal of nanoscience and nanotechnology, 10(11), 7183-7187, (2010).

Microwave processing methods have greatly decreased the times needed to topochemically modify layered perovskites. Reactions involving ion exchange, intercalation, grafting, and exfoliation have all been impacted. Here we find that quality products can often be obtained in hours instead of days. In this presentation, methods and products will be highlighted in terms of the utility of this approach, not only in the formation of bulk materials, but also in the production and modification of exfoliated nanosheets.

Metal halide semiconductors while highly desirable for a host of applications, present several challenges that remain unresolved, including the ability to control their dimensionality at the molecular level (orientation and mode of attachment), matching of phonon band structure in inorganics with discrete vibrations within the molecule, and tuning energetic offsets for effective charge and spin transfer. Each of these parameters ultimately governs charge transport, the preservation of coherence, and energy transfer.
In this talk, I will discuss fundamental guidelines for the design of dimensionally stable metal halide heterostructures with controlled morphology and interfaces that serve as conduits for deterministic and coherent spin and charge transfer. I will divide my talk into two different sub-topics, i.e., (1) how matching phonon structure across interfaces through control of mode and site of attachment of molecules to surfaces allow for efficient coherent charge transfer and (2) how topologically protected electronic states can be defined by 2D assembly of pi-conjugated molecules and layered metal halide materials on surfaces for spin-coherent electronic transport. Here, I will emphasize the role that dimensionality plays in modulating the spin and charge properties of metal halide heterostructures.

The unique structure diversity, tunable compositions, and physical-chemical properties of high entropy oxides (HEOs) attract wide research interests. At the same time, there are vast economic and environmental needs for obtaining stable, high-capacity, and low-cost catalysts for automotive and industrial exhaust applications; in particular, high surface areas and high thermal stability of oxygen storage capacity catalysts are often incompatible. Here, an eco-friendly soft-templating route is applied to form mesoporous HEO spinel nanostructures (MCo₂O₄, M= Mn²⁺, Fe²⁺, Ni²⁺, Mg²⁺, Zn²⁺, etc.), in which high surface area, thermal stability, and compositionally tuned catalysis behavior can be simultaneously achieved. The structural order/disorder in the HEO spinel lattice can be achieved through M cation selection and the morphology (ranging from highly faceted octahedron to highly mesoporous nanocrystalline agglomerates) can be tuned through the selection of templates and reaction conditions. We apply a wide array of characterization techniques, including electron microscopy, and neutron and X-ray total scattering probes, to correlate these characteristics with thermal and catalytic properties. Identifying the role of specific lattice defects, chemical short-range order, and nanoscale heterogeneity in this MCo₂O₄ spinel family will promote design principles and strategies for HEO nanostructured catalysts.

Ternary nitrides are an emerging class of inorganic materials for use in a variety of optical, electronic, and refractory applications. In preparing inorganic nitrides, selection of precursors and reaction conditions is dependent on controlling the reactivity of the nitride anion, as decomposition to yield N₂ is thermodynamically favorable at high temperatures used in traditional ceramic processes. While high pressures or reactive environments like ammonia can change the chemical potential of nitrogen to favor a nitride product, reactions that occur under low-temperatures and mild reaction conditions are desired. Here we present on low-temperature ion-exchange reactions that yield magnesium metal nitrides and zinc metal nitrides at low-temperatures and pressures. First, we report on the synthesis of rocksalt MgZrN₂, rocksalt Mg₂NbN₃, and hexagonal MgMoN₂ using magnesium nitrochloride and the corresponding transition metal halides as precursors. Key to these preparations is a low-temperature anneal (300-450 ^○C), which calorimetry experiments show results in Mg-M-N product formation. Then, we present on additional experiments using these magnesium metal nitrides as a structural template to perform selective ion-exchange reactions with zinc halide precursors to yield metastable zince metal nitrides: rockalt Zn_xMg_1-xZrN₂, rocksalt Zn_xMg_2-xNbN₃, and layered Zn_xMg_1-xMoN₂. Overall, these low-temperature ion exchange reactions hold promise to increased discovery of new ternary metal nitrides.

The electrochemical properties of a battery cathode material depend on the structural and associated thermodynamic changes that are induced by ion intercalation and deintercalation. Despite the prevalence of LiMPO₄ cathodes (M represents Mn, Fe, Co, Ni, or a mixture of multiple of these elements), some aspects of local structural characteristics and thermodynamics are not thoroughly understood in these materials. Li_xFePO₄ undergoes a transformation from a two-phase mixture of triphylite and heterosite to a distinct solid solution phase at elevated temperatures. In this work, the phase transformation in Li_xFePO₄ x=0.6 is investigated using neutron powder diffraction, inelastic neutron scattering, and calorimetry. We report contributions to the vibrational and configurational entropy as a function of temperature, both above and below the phase transformation. Results are compared to first-principles calculations of phonon spectra. New insights into the how the electronic and ionic conductivity differs in each phase will be discussed.

Nanotechnology has the potential to upgrade the oil and gas industry within different areas such as exploration, drilling, production, and enhanced oil recovery (EOR). For instance, the use of nanoparticles as an EOR method exhibit many advantages since:
The size and shape of nanoparticles can be easily modified during fabrication.
Nanoparticles' surface chemical properties can be easily adapted to turn them hydrophobic or hydrophilic and modify the chemical interaction with specific types of surfaces.
Nanoparticles may resist more complex conditions in comparison with conventional EOR chemical methods.
During the last decade, numerous studies have been conducted employing nanoparticles for EOR purposes. Although different types of nanoparticles, both organic and inorganic, show a significant increase in the additional recovery factor (RF) of crude oil, these results are encouraging, specifically for mature fields.

This work presents the synthesis of inorganic nanoparticles and their performance evaluation in oil displacement tests under different conditions: water-oil ratios, the concentration of nanoparticles, temperature, and salinity of various synthetic brines. Tests were carried out with sandstone to evaluate the interactions between rock-nanoparticles and brine-nanoparticles. During these tests, the nanoparticles displaced up to 50 % of the oil, compared to 26 % of oil displaced by synthetic brines. Additionally, we demonstrated that nanoparticles are helpful to reduce the water-oil interfacial tension, resulting in the formation and stabilization of Pickering-type emulsions, which have shown the ability to be formed in-situ and improve oil recovery in crude oil reservoirs. The results opened the door to future experiments for assessing the wettability alteration promoted by nanofluids and the efficiency of displacement of oil contained in a porous media.

The MMn(Bi,Sb)₂ (M=rare earth or alkaline earth) family has emerged as a fertile ground for exploration of topological semimetals. Topological semimetals are charge-balanced compounds that host Dirac or Weyl fermions and with electronic excitations protected by topology. Several members of this family have been explored, such as (Ca,Sr,Ba,Eu,Yb)Mn(Sb,Bi)₂, all with divalent oxidation state of the rare earth or alkaline earth metal. Here, we report on the discovery of a new member of the family, SmMnBi₂, and compare its structure to other compounds in the family. Surprisingly, despite the near-identical ionic sizes of Eu²⁺ and Sm²⁺, the structural layers in SmMnBi₂ differ from that of EuMnBi₂.

Na_xSi₁₃₆ (0 ≤ x ≤ 24) materials with the clathrate-II crystal structure provide a unique opportunity to investigate the direct effect of guest content on physical properties of intermetallic clathrates in a chemically simple system with only two elements. The preferential filling of two framework cages with very different sizes is accompanied by very different guest-framework interactions, resulting in a non-monotonic structural response to cage filling that is correlated with a metal-insulator transition near x = 8. Temperature-dependent single-crystal and powder X-ray diffraction experiments show that the room temperature linear coefficient of thermal expansion increases by nearly a factor of 3 upon filling, attributed to an increase in the anharmonicity in the interatomic interactions. Furthermore, negative thermal expansion, present at low temperature (T < 150 K) in Si₁₃₆ and Na_xSi₁₃₆ at lower Na contents, disappears for x > 16. Extremely large atomic displacement parameters obtained from structure refinements, as well as dynamic instability observed in first principles calculation of phonon dispersion, suggest the Na guest in the larger cage is displaced off-center, but the exact nature of the off-centering and dynamical behavior remains difficult to elucidate. Thus, modeling and understanding the physical mechanisms underlying the effects of guest content on electrical, thermal, and mechanical properties of these silicon clathrates is non-trivial.

Among the versatile mixed anion compounds [1], oxychalcogenides are emerging as promising alternative candidates for a variety of applications including for energy. Recently, they have been pointed as interesting systems for mid IR nonlinear optical (NLO) properties [2]. In particular, strong second harmonic generation (SHG) responses are found for NLO compounds with functional groups composed of mixed anions such as in BaGeSe₂O with GeO₂Se₂ tetrahedra [3] (compared to its oxide analogue with GeO₄ tetrahedra). Here we will discuss, through several structure types based on thiovanadates V(O,S)₄ building blocks, the possibility to tune the symmetry and engineer the band gap. Ba₅(VO₂S₂)₂(S₂)₂ [4] with third-harmonic generation properties and Ba₇S(VOS₃)₂(S₂)₃ and its selenide-derivatives [5] will be discussed with support of density functional theory (DFT). We will see that the presence of Q–Q bonds (Q= chalcogenide) lead to original lattices with multiple anionic species and add greater structure and electronic structure versatility to design functional materials. On another hand, we will present unprecedented oxysulfide building block for SHG active materials and give a comprehension of the SHG response mechanism with support of DFT calculations [6].
References :
[1] Kageyama, H. et al. Nature Communications 2018, 9 (1), 772.
[2] Li, Y. Y. et al. Cryst. Growth Des. 2019, 19 (7), 4172–4192.
[3] Liu, B. et al. Chemistry of Materials 2015, 27 (24), 8189–8192.
[4] Almoussawi, B. et al. Inorganic Chemistry 2020, 59 (9), 5907–5917.
[5, 6] submitted (2021)

Many of the most widespread and commercially successful materials in Li-ion batteries are oxides. Nevertheless, some of the best performing materials contain elements from elsewhere in the periodic table (e.g. chalcogenides, pnictides). Although halide salts have long been employed as electrolytes in solution or in the molten state, there are few examples of halides as solid-state cationic conductors and until recently, still fewer in terms of hydrides.

This contribution focuses principally on the possibilities for designing fast Li-ion conducting (pseudo-)halides as potential electrolytes in all-solid-state batteries. In switching the materials design emphasis from cations to anions, it will be shown how both static and dynamic disorder might be exploited to enhance Li-ion transport and how diffusion pathways might be modified in terms of anion geometry and electronegativity (polarizability). Concepts of lattice “softness” and complex anion dynamics become crucial components of such a design strategy. Synthesis (often of metastable solids) can be challenging, but mechanochemistry and other “soft” chemical approaches, can provide a means to access such materials, while facilitating nanostructuring and generating defects.

This contribution covers several exemplar systems of promising complex hydride and halide Li fast-ion conductors, including Li(BH₄)_1-yX_y, LiAlX₄ and LiCe(BH₄)₃X; X = Cl, Br, I, among others. In each case a raft of techniques including neutron diffraction and muon spin relaxation spectroscopy have been fundamental in elucidating the structure-property relationships in these systems. A synergic combination of experiment and computational calculations are beginning to shed light on the mechanisms for Li-ion conduction in these families of non-oxide materials and their prospects as solid state electrolytes will be evaluated.

Group VI metal oxides featuring MoO₄^–2 and WO₄^–2 anions are ideal semiconducting host materials for thermosensitive phosphors activated by Mn⁴⁺ and/or rare-earth metals. A distinct feature of these materials is their structural diversity in terms of metal coordination and connectivity, which makes them an ideal platform to exercise compositional control over their temperature-dependent luminescence response. In this talk, I will highlight the potential of molybdate and tungstate phosphors as luminescent thermometers. I will show that selected Mn⁴⁺-activated metalates serve as bandshift thermometers with a temperature sensitivity comparable to that of quantum dots but with a much broader usable temperature range. Further, I will show how the crystal-chemistry of group VI metalates may be exploited to generate thermochromic phosphors via codoping Mn⁴⁺–Tb³⁺ pairs. Findings presented in this talk expand the scope of applicability of Mn⁴⁺-activated thermosensitive phosphors beyond ratiometric bandshape and lifetime thermometry. They also provide structural guidelines for discovering and screening novel oxide hosts for dual-activator luminescent thermometers.

We outline a state-of-the-art approach for reconstructing the orientation distribution function (ODF) of materials from their scattering anisotropy using real spherical harmonics (RSH) expansion. Without assuming the functional form of ODF, we perform the reconstruction through the RSH decomposition of their scattering intensity and ODF subject to certain constraints from symmetries and orientation order parameters. We demonstrated its validity by the quantitative agreement between case studies of molecular dynamics simulations and numerical computations. Our proposed method is applicable for general materials subject to various external conditions. Material’s properties usually reflect on the coarse-grained averages of the relative orientation and separation at molecular levels over a characteristic length scale. One of the well-known examples is the magnetic response of materials is the ensemble average of magnetic moments over all the possible configuration distributions in phase space. Knowing how the molecular orientation distributes enables us to investigate the fundamental mechanism between the microscopic variables and macroscopic properties of interest in response to the applied fields. It can facilitate us with materials design and characterization for its practical use under different conditions. These can be exemplified by the magnetization alignment induced by the applied magnetic field or the orientational ordering of anisotropic nanoparticles in composites undergoing mechanical deformation. Typically, the microscopic information of material’s orientation is conceived in its corresponding scattering spectra, which is an ensemble average in momentum space. Under the external fields without rotational symmetry, the constituent of materials would possess a preferred orientation, which will reflect on the anisotropy of their scattering signature. Therefore, how to reconstruct the ODF of materials from their scattering anisotropy is inevitably important.

We present a density functional theory study of the interplay between multiple hidden and established charge, magnetic and structural orders in 5d¹ double perovskites. The ordering of structural distortions, magnetic dipole moments, atomic orbital occupancies, and electronic charges is widespread in materials and leads to well-known behaviors such as changes in the crystal structure, the onset of ferromagnetism, or metal-insulator transitions. While such orderings have been extensively investigated, there remain many materials that show intriguing changes in behavior indicating the onset of order, but for which the nature of the order is not yet known. Double perovskite oxides (halides) with 5d1 transition magnetic ion have been of particular interest recently to explore these multipolar hidden orders which stem from their unique crystal chemistry and the interplay of spin-orbit coupling, crystal-field splitting, and electron-electron repulsion. Here, we present a density functional theory study of multipolar ordering (e.g. charge quadrupoles and magnetic octupoles) and their coupling with the dipolar magnetic and the crystal structure of two 5d¹ double perovskites, Ba₂MgReO₆ and Cs₂TaCl₆. Both of these compounds are spin-orbit-driven Mott insulators with a symmetry-lowering structural phase transition at low temperatures driven by the ordering of charge quadrupoles. Our primary goal is to provide an understanding of the interplay between the structural distortions, the charge quadrupole ordering, and the magnetic dipole ordering in these compounds. In addition to shedding light on the detailed behavior of Ba₂MgReO₆ and Cs₂TaCl₆, our analysis provides a novel insight into the crystal structure-multipolar order relationship of 5d¹ double perovskites.

Porous crystalline materials such as zeolites and metal-organic frameworks (MOFs) have attracted wide attention due to their well-defined porous structures and high surface area, which are useful in gas storage and separation, ion exchange and conduction, and heterogeneous catalysis. Polyoxometalates (POMs) are nano-sized metal-oxide molecular anions with unique catalytic, electrochemical, magnetic, and luminescent properties and have stimulated research in broad fields of science. Recently, the incorporation of POMs in nano-sized cavities of MOFs has been reported; these POM-MOFs show unique functions such as oxidative removal of hydrogen sulfide, selective adsorption of cationic dyes, and visible-light-driven hydrogen production. In contrast, we have reported that the complexation of POMs with molecular cations with appropriate elements, charges, sizes, symmetry, ligands, etc., results in the formation of voids and channels in the crystal lattice. The properties of porous ionic crystals (PICs) based on POMs can be summarized as follows. (a) POMs show reversible redox properties, which can lead to the formation of “redox-active” PICs. (b) Guest binding and catalytically active sites can be incorporated beforehand into the ionic components. These functions can be maintained and utilized in the PICs since the ionic components still exist as discrete molecules in the crystal lattice. (c) The ionic components create strong electrostatic fields at the internal surfaces of PICs, which are suitable for accommodating and stabilizing polar guests and reaction intermediates. (d) POMs can transport protons as counter cations efficiently because the negative charge of POMs is smeared over the external oxygen atoms decreasing the effective surface charge density. These properties lead to interesting functions in selective ion-uptake/exchange, proton conduction, heterogeneous catalysis, formation of luminescent mixed-valence metal clusters, etc., which are unique to PICs.^1-12
Recent papers: (1) ACS Appl. Mater. Interfaces 13, 19138 (2021), selected as ACS editor’s choice, (2) Nanoscale 13, 8049 (2021), selected as backside cover. (3) Eur. J. Inorg. Chem. 1531 (2021), selected as frontside cover. (4) Chem. Lett. 50, 21-39 (2021), highlight review. (5) Dalton Trans. 49, 10328 (2020), selected as backside cover. (6) Langmuir 36, 6277 (2020). (7) Chem. Sci. 10, 7670 (2019), selected as frontside cover. (8) ChemCatChem 11, 3745 (2019), selected as frontside cover. (9) Communications Chemistry Article number: 9 (2019). (10) Inorg. Chem. 56, 15187 (2017). (11) Angew. Chem. Int. Ed. 55, 3987 (2016). (12) Chem. Mater. 27, 2092 (2015).

Efficient transfer of charges at the interface of transition-metal and semiconductor can improve efficiency of photocatalyst. Herein, we examined the photoactivity of Mn and Zn-doped MIL-88A for CO₂ reduction and N₂ fixation by water under UV-visible irradiation and ambient conditions. Gaseous-phase photo-reduction of CO₂ generated CO, H₂, and CH₄. Apparent quantum yield of Zn-MIL-88A was found to be 2.16-folds higher than that of pristine MIL-88A catalyst. Also, the highest NH₄⁺ generation rate was for Zn-MIL-88A catalyst. Dissecting the underlying mechanism discloses that electrons efficiently transferred from doped transition-metals to the Fe-O cluster of MIL-88A. The hydrophilic surface of the tested catalysts in a tri-phase photocatalyst system led the process to be controlled by mass-transport; which governed the product distribution and limited the kinetics of the process. Experimental results also unveil that the addition of transition metals improved stability, charge-separation, and efficiency of the resulting photocatalysts.

In steelmaking processes that refine pig iron into tough steel, calcined lime (CaO) is added to remove unwanted components, such as Si, P, and S, and steelmaking slag is produced as a by-product. This slag consists primarily of CaO and SiO₂ and is effectively used as recycled materials for construction and civil engineering. However, part of the CaO in the slag remains in the unreacted form, i.e. as free lime, which expands to about double volume by hydration. Therefore, fresh slags that contain free lime must undergo aging. In general, slags containing high amounts of free lime have higher expansibility than their counterparts, which have lower CaO contents. The expansibility, however, varies with the particle size, physical state of the free lime, and the corresponding chemical composition. Therefore, estimating the expansibility solely as a function of the free lime content is difficult. In this study, to promote the utilization of steelmaking slag, the hydration reactivity and electronic state of free lime were observed. It was confirmed by X-ray diffraction (XRD) and thermal analysis that the hydration reactivity decreases when CaO forms a lime-phase solid solution with FeO, and it was revealed by X-ray photoelectron spectroscopy (XPS) observations and DV-Xα calculation that the decrease in reactivity occurs due to deepening of the energy levels of the electron orbitals. The morphology and hydration reactivity of the free lime contained in a synthesized slag were observed by XRD and electron probe microanalysis. Fe-containing phases and double oxides were found to be adjacent to free lime; the former is considered to be related to the formation of a solid solution with FeO, and the latter closely surrounds the free lime and shields it from the outside; thus, both contribute to suppressing the hydration reaction. Furthermore, it was found that simply mixing lime with double oxides suppressed the hydration reaction. XPS and thermal analysis confirmed that a chemical change occurred on the surface of the free lime; that is, the energy levels of the electron orbitals are deepened and the hydration reaction is suppressed due to chemical interactions with the double oxides to stabilize the lime surface. It was clarified that in order to effectively promote the hydration of free lime, it must be exposed by breaking the structure of the slag that protects the free lime and suppresses hydration.

The electronic properties of rare-earth elements, RE are dominated by the 3+ oxidation state, but 2+ can be also stabilized for Sm, Eu, and Yb due to their comparable energy ^[1]. As a result, mixed valency phenomena are encountered in compounds of these rare-earths (Kondo insulators, heavy fermions) with the average RE valence strongly affected by changes in external stimuli (temperature, pressure). Research in elastic, conducting, optical, and magnetic properties triggered by such valence changes remains at the frontier of solid-state chemistry.
The RE fullerides with stoichiometry, RE_2.75C₆₀ (RE=Sm, Yb) constitute an intriguing class of electronic systems due to the strong electronic coupling between the highly-correlated mixed-valence RE cation and C₆₀ anion sublattices. Past work pointed towards strong coupling of lattice and electronic degrees of freedom with the occurrence of pressure-induced lattice collapse and concomitant insulator-metal transition around 4 GPa ^[2,3]. However, to-date there has been no direct study of the RE electronic states – all information about valence changes has come indirectly from diffraction and Raman spectroscopic measurements.
Here we report the results of element-specific synchrotron X-ray absorption spectroscopic (XAS) measurements of the average Sm valence in binary and ternary Sm fullerides, authenticating the mixed valence character of the materials for the first time. In addition, we directly address the evolution of the Sm electronic states both by chemical pressure induced by progressive introduction of smaller-in-size valence-precise Ca²⁺ ions and by external physical pressure ^[4,5]. Following this spectroscopic work, we address the structural behavior of this family of materials by high-resolution synchrotron X-ray powder diffraction measurements at elevated pressures – the observed lattice collapse with increasing pressure sensitively depends on the level of Ca²⁺ substitution and perfectly mirrors the Sm valency transition authenticated by synchrotron XAS. The pressure-induced phase transition is isostructural but is accompanied by well-defined renormalization of the rare-earth metal positions and their shift towards high-symmetry positions. The results provide a rationalization of the pressure-induced drastic changes in structural, conducting and optical properties of RE fullerides that are coupled with the extreme fragility of the rare-earth valence.

[1] P. Strange, A. Svane, W. M. Temmertman, Z. Szotek & H. Winter, Nature, 399, 756-758 (1999)
[2] J. Arvanitidis, K. Papagelis, S. Margadonna, and K. Prassides, Dalton Trans., 19, 3144-3146 (2004)
[3] S.M. Souliou, J. Arvanitidis, D. Christofilos, K. Papagelis, S.Ves, G.A. Kourouklis, K. Prassides ,Y. Iwasa and K. Syassen, High Pressure Research, 31, 13-17 (2011)
[4] N. Yoshikane, T. Nakagawa, K. Matsui, H. Yamaoka, N. Hiraoka, H. Ishii, J. Arvanitidis, K. Prassides*, J. Phys. Chem. Solids 150, 109822/1-6 (2021).
[5] N. Yoshikane, K. Matsui, T. Nakagawa, A. G. V. Terzidou, Y. Takabayashi, H. Yamaoka, N. Hiraoka, H. Ishii, J. Arvanitidis, K. Prassides*, Mater. Chem. Front. 4, 3521-3528 (2020).

The discovery of high temperature superconductivity in cuprates has spurred the search for novel low dimensional cuprates such as spin ladder compounds. On the other hand, less attention has been paid for the three dimensional counterpart, the perovskitetype cuprates ACuO3, presumably because of the difficulty in high pressure synthesis. In this talk, a novel orthorhombic perovskite PrCuO3 obtained by high pressure synthesis will be introduced[1]. Synchrotron x-ray diffraction and x-ray absorption spectroscopy reveal that PrCuO3 consists of quasi-one-dimensional chains of Cu-O with square-planar coordination, and the oxidation state of Cu ions is about 2.2+. The formation of the Cu-O chain reflects the cooperative Jahn-Teller distortion of nearly divalent Cu ions. Upon the La substitution for Pr, we found the abrupt change in structure and electrical conductivity, from the orthorhombic semimetal to rhombohedral metal, which is accompanied by the AB site charge transfer. This system offers a unique opportunity to explore novel quantum phases of correlated electrons as the simplest one-dimensional counterpart of the high temperature superconducting cuprates. In addition, we will introduce a computational structure prediction for novel oxygen-deficient perovskite-type cuprates on the basis of the density functional theory and Bayesian optimization[2].

References
[1] M. Ito, H. Takahashi, H. Sakai, H. Sagayama, Y. Yamasaki, Y. Yokoyama, H. Setoyama, H. Wadati, K. Takahashi, Y. Kusano and S. Ishiwata, Chem. Commun. 55, 8931 (2019).
[2] A. Seko and S. Ishiwata, Phys. Rev. B 101, 134101 (2020).

Perovskite-type oxides(ABO3) are quite common for various Ferroelectric, Piezoelectric, Conducting Electrode and/or even Superconducting oxides. However, it has not been clear why they will take perovskite-type structure(s) . For the stability of Perovskite Lattices, classic Tolerance Factors based upon ion packing model have been considered even now. One of the present authors proposed another concept: Madelung lattice energy should be the key factor of the Lattice and Valence Stability.^1,2) The total Madelung Lattice energy(U) could be considered by the summation of Sub-Lattices Energies of A, B and O sites. Since they are given each lattice site potential (Φ) and its valence(q): that is, U = ΦA x qA + ΦB x qB + 3ΦO x qO. Using a recent Program:VESTA³⁾,we have confirmed 1) U_ABO3 is always larger than the summation of U_AO + U_BO2 or U_AO1.5 + U_BO1.5 . 2) U_ABO3 become larger when valence pairs are shifted from 3+ and 3+ to, 4+ and 2+ then 1+ and 5+ or 0+ and 6+. 3) Furthermore, those large B Sublattice potential can be strongly related to high valence state of B ions in Perovskite ABO3. Since the ionization potential [loss] can be compensated by high lattice site potential [gain], high valence ion(s) can be stabilized in B site with high lattice-site potential. For example, Ce4+, Pr4+, and Tb4+ can be stabilized in a Fluorite lattice, and more stabilized in the B site of Perovskite ABO3 lattice. Un-common high-valence ions like Co3+, Ni3+, Fe4+ in the B-sites of the Perovskite ABO3 lattices can be explained similarly. On the other hand, those high-valence ions cannot be stabilized in the A site of Perovskites nor various A2O3, like A-type and C-type M2O3,or corundum, lattices thus they take rather common valence states in those cases. The Madelung Lattice energy(U) is Enthalpy at 0 K,thus it consists of ca. 90% of total free energy(G) of the compound. Therefore the valence stability of certain ions in particular lattices is directly related to the Madelung self-site potential(Φ) of the lattice site. The entropy term by Temperature and P_O2 can contribute < 10% of U at 1000K. Therefore it might be rather minor effects for those internal lattice potential energy. Vacancy formation energies can also be estimated by Madelung Lattice Site Potentials. More precise data can be calculated by recent First Principle Calculation using several additional parameters, but they should include several hypothetical parameters and much longer times for calculation.

Reference
[1] M. YOSHIMURA,et al.: Stabilization of High valence States of Ions at B site of Perovskite Structure ABO3: Electrostatic Potential in Perovskite Structure; Bull. Tokyo Inst. Tech., 120, 1327, (1974).
[2] M. YOSHIMURA,Valence Stability of Rare Earth Ions,(in Japanese), Chapter 34,pp 851-862, Kidorui (Rare Earth) Handbook, Edited by G. Adachi, NTS Book Co. Ltd. Tokyo,2008.
[3] M.Yoshimura and K. Sardar, Revisiting Valence Stability and Preparation of Perovskite-type Oxides ABO3 with the Use of Madelung Electrostatic Potential Energy and Lattice Site Potential, RSC Advance.2021,11,20737-20745.