Yue Chen, University of Hong Kong
Thomas Hammerschmidt, Ruhr-Universität Bochum
Alexey Kolmogorov, Binghamton University
Kuo Li, Center for High Pressure Science and Technology Advanced Research
Jung-Fu Lin, The University of Texas at Austin
CM1.1: Superconductivity Under Pressure
Tuesday AM, April 18, 2017
PCC North, 100 Level, Room 125 A
11:30 AM - *CM1.1.01
Superconductivity in Topological Compounds Tuned via Pressure
Changqing Jin 1 Show Abstract
1 , Institute of Physics, Chinese Academy of Sciences, Beijing China
Topological insulators form very interesting new quantum category[1~6] since featured with gapped bulk state but a gapless metallic edge state. Right after theoretically predication of 3 dimensional topological insulators, ARPES experiments show strong evidences by observation of Dirac cone at surface electronic structure. Topological superconductor, as an analog of topological insulator is one of most excited physical properties of topological quantum matters. Superconductivity was first reported in Cu intercalated Bi2Se3 topological compound. In addition to chemical doping, pressure can be a direct physical measure to modify electronic structures through changing the atomic distance with the advantages of free of defects or impurities inherent to doping. Here we introduce our recent work on pressure induced superconductivity in topological materials or related system with strong spin orbital coupling behavior including Weyl semimetals & Rashiba compounds[6~8].
Acknowledgments: This work was supported by NSF & MOST of China.
Email:Jin@iphy.ac.cn; Group web site: http://uhp.iphy.ac.cn
1. H. J. Zhang et al., Nature Physics 5, 438 (2009).
2. Y. Xia et al., Nature Physics 5, 398 (2009).
3. Hsin Lin et al., NATURE MATERIALS 9, 546(2010)
4. Y. S. Hor et al., Phys. Rev. Lett.104, 57001 (2010).
5. A. Wray et al., NATURE PHYSICS 7, 32(2011)
6.J. L. Zhang et al., Proc. Natl Acad. Sci. 108, 24 (2011)
7. J. Zhu et al , Scientific Reports | 3 : 2016(2013) | DOI: 10.1038/srep02016
8. P.P. Kong et al., Scientific Reports | 4: 6679 (2014)
12:00 PM - CM1.1.02
Superconductivity and Unexpected Chemistry of Germanium Hydrides under Pressure
M. Mahdi Davari Esfahani 1 2 , Artem Oganov 1 2 , Hayiang Niu 1 2 , Jin Zhang 1 2 Show Abstract
1 Department of Geosciences, State University of New York, Stony Brook, Stony Brook, New York, United States, 2 Institute for Advanced Computational Science, State University of New York, Stony Brook, Stony Brook, New York, United States
Following the idea that hydrogen-rich compounds might be high-Tc superconductors at high pressures, and the very recent breakthrough in synthesizing hydrogen sulfide with record-high Tc = 203 K, ab initio evolutionary algorithm for crystal structure prediction was employed to find stable germanium hydrides. In addition to the earlier structure of germane with space group Ama2, we propose a more energetically favorable structure, with space group C2/m at pressures above 280 GPa (with inclusion of zero point energy). Our calculations indicate metallicity of the new C2/m phase of germane with superconducting Tc = 67 K at 280 GPa. Germane is found to exhibit thermodynamic instability to decomposition to hydrogen and the new compound Ge3H11 at pressures above 300 GPa. Ge3H11 with space group I-4m2 is found to become stable at above 300 GPa with Tc = 38 K. We find that the pressure-induced phase stability of germanium hydrides is distinct from its analogous isoelectronic systems, e.g., Si-hydrides and Sn-hydrides. Superconductivity stems from large electron-phonon coupling associated with the wagging, bending and stretching intermediate-frequency modes derived mainly from hydrogen.
12:15 PM -
12:30 PM - CM1.1.04
Superconductivity and Hybrid Soft Modes in 1T-TiSe2
Frank Weber 1 , Stephan Rosenkranz 2 , Roland Hott 1 , Rolf Heid 1 , Diego Zocco 1 , Michael Maschek 1 , Said Ayman H. 3 , Ahmet Alatas 3 , Goran Karapetrov 4 , Shan Zhu 5 , Japser van Wezel 5 Show Abstract
1 Institute for Solid State Physics, Karlsruhe Institute of Technology, Karlsruhe Germany, 2 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 3 Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois, United States, 4 Department of Physics, Drexel University, Philadelphia, Pennsylvania, United States, 5 Institute for Theoretical Physics, University of Amsterdam, Amsterdam Netherlands
The competition between superconductivity and other ground states of solids is one of the challenging topics in condensed matter physics. Apart from high-temperature superconductors [1,2] this interplay also plays a central role in the layered transition-metal dichalcogenides, where superconductivity is stabilized by suppressing charge-density-wave order to zero temperature by intercalation  or applied pressure [4-7]. 1T-TiSe2 forms a prime example, featuring superconducting domes on intercalation as well as under applied pressure. Here, we present high energy-resolution inelastic x-ray scattering measurements of the CDW soft phonon mode in intercalated CuxTiSe2 and pressurized 1T-TiSe2 along with detailed ab-initio calculations for the lattice dynamical properties and phonon-mediated superconductivity . We find that the intercalation-induced superconductivity can be explained by a solely phonon-mediated pairing mechanism, while this is not possible for the superconducting phase under pressure. We argue that a hybridization of phonon and exciton modes in the pairing mechanism is necessary to explain the full observed temperature-pressure-intercalation phase diagram. These results indicate that 1T-TiSe2 under pressure is close to the elusive state of the excitonic insulator.
 B. Keimer, S. A. Kivelson, M. R. Norman, S. Uchida, and J. Zaanen, Nature 518, 179 (2015).
 D. C. Johnston, Advances in Physics 59, 803 (2010).
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 A. Kusmartseva, B. Sipos, H. Berger, L. Forró, and E. Tutiš, Physical Review Letters 103, 236401 (2009).
 Y. I. Joe et al., Nature Physics 10, 421 (2014).
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 M. Maschek et al., arXiv:1610.01041 (2016).
12:45 PM - CM1.1.05
A Combined Theoretical and Experimental Study on the High-Pressure Phases in the Sn-Se System
Hulei Yu 1 , Wenxin Lao 1 , Lijuan Wang 2 , Kuo Li 2 , Yue Chen 1 Show Abstract
1 , The University of Hong Kong, Hong Kong Hong Kong, 2 , Center for High Pressure Science and Technology Advanced Research, Beijing China
A new tin selenium compound with an unexpected 3:4 stoichiometry is predicted under high pressures from evolutionary algorithms and density functional theory. Corresponding high-pressure experiments were performed and the Sn3Se4 compound was successfully synthesized. We find that Sn3Se4 exhibits very different behaviors comparing to the well-known SnSe and SnSe2. As Sn has a +2 or +4 oxidation state and Se has an oxidation number of -2, the existence of Sn3Se4 with a counter-intuitive stoichiometry suggests novel chemical bonding under high pressures. Electron density and Bader charge analyses were carried out and a mixed character of chemical bonds was revealed. A superconducting transition is found under much lower pressures than previously reported for the Sn-Se system due to the lower metallization pressure of Sn3Se4. Our work suggests potential existence of undiscovered compounds with counter-intuitive stoichiometries and novel properties in the IV-VI group.
CM1.2: Superhard Materials and Mechanical Behaviors
Tuesday PM, April 18, 2017
PCC North, 100 Level, Room 125 A
3:00 PM - *CM1.2.01
Novel Superhard Materials in Light Element B-C-N-O System
Duanwei He 1 Show Abstract
1 , Sichuan University, Chengdu China
Superhard materials are usually composed of light elements such as boron, carbon, nitrogen, and oxygen. The intrinsically strong and directional covalent bonds of these light elements lead to tight, three-dimensional networks with extreme resistance to external shear. Now, diamond, cubic boron nitride (cBN), and their composite materials, the well known superhard materials, have widely been used to prepare high-pressure anvil, explore oil-gass and geological, and cutting concrete and steel, polishing stones, machining, and honing. Compared with the traditional hard materials (WC, Al2O3 and SiC), diamond, cBN, and their composite materials possess many advantages in practical application, such as high efficiency, energy conservation and environmental protection. However, each of them has inherent limitations. Diamond easily oxidizes at relatively low temperatures and reacts with iron group metals and thus not suitable for high-speed cutting and polishing of ferrous alloys. While cBN—the super abrasive of choice for machining hard ferrous steels—has superior thermal stability and reaction resistance (1376 K for cBN and 953 K for diamond), its hardness is much lower than that of diamond (47 GPa for cBN and 85 GPa for diamond). Therefore, the growing industrial demand for machining hard ferrous alloys and ceramics has stimulated a search for novel superhard phases having high hardness and high thermal stability.
This work presents the progresses of high-pressure and -temperature synthesis of novel superhard materials in light element B-C-N-O system, inclosing micron/nano polycrystalline diamond, cBN, B6O, and diamond-cBN alloy. In the recent years, through optimizing the two stage 6-8 type large cavity static high pressure device based on the domestic cubic pressure, we can synthesis the samples in centimeter at above 15 GPa and 2300 K, and their performances in cutting concrete and steel have been tested.
3:30 PM - *CM1.2.05
A Statistical Learning Model for Elastic Moduli of Inorganic Compounds—Application to Discovery of New Superhard Materials
Maarten de Jong 1 3 , Randy Notestine 4 , Ji Wei Yoon 1 , Anthony Gamst 4 , Mark Asta 1 2 Show Abstract
1 Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States, 3 , Space Exploration Technologies, Hawthorne, California, United States, 4 Computational and Applied Statistics Laboratory, San Diego Supercomputer Center, University of California, San Diego, La Jolla, California, United States, 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Machine or statistical learning (SL) techniques are finding growing use in applications related to the acceleration of materials discovery and design. Such pursuits benefit from pooling training data across, and thus being able to generalize predictions over, k-nary compounds of diverse chemistries and structures. In this talk we describe work involving the development of a SL framework  that addresses challenges in materials science applications, where datasets are diverse but of modest size, and extreme values are often of interest. Ingredients include the application of power or Hölder means to construct descriptors that generalize over chemistry and crystal structure, and the incorporation of multivariate local regression within a gradient boosting framework. The approach is demonstrated by developing SL models to predict bulk and shear moduli (K and G, respectively) for polycrystalline inorganic compounds, using 1,940 compounds from a growing database  of calculated elastic moduli for metals, semiconductors and insulators, which is available through the open Materials Project database. The SL model is employed to screen for superhard materials employing a recently proposed empirical relationship between hardness and the bulk and shear moduli . Several known superhard materials are identified by this approach, as well as a set of new compounds that to our knowledge have not been previously reported. This work was intellectually led by the Department of Energy Basic Energy Sciences program - the Materials Project - under Grant No. EDCBEE.
 M. de Jong, W. Chen, R. Notestine, K. Persson, G. Ceder, A. Jain, M. Asta and A. Gamst, “A Statistical Learning Framework for Materials Science: Application to Elastic Moduli of k-nary Inorganic Polycrystalline Compounds,” Sci. Rep. 6, 34256; doi: 10.1038/srep34256 (2016).
 M. de Jong, W. Chen, T. Angsten, A. Jain, R. Notestine, A. Gamst, M. Sluiter, C. K. Ande, S. van der Zwaag, J. J. Plata, C. Toher, S. Curtarolo, G. Ceder, K. A. Persson and M. Asta, “Charting the Complete Elastic Properties of Inorganic Crystalline Compounds,” Scientific Data 2:150009 (2015).
 H. Niu, X.-Q. Chen, P. Liu, W. Xing, X. Cheng, D. Li and Y. Li, “Extra-electron induced covalent strengthening and generalization of intrinsic ductile-to-brittle criterion,” Sci. Rep. 2, 718; doi: 10.1038/srep00718 (2012).
4:30 PM - CM1.2.06
Combined Experimental and First Principles Dft Study of the Equation of State and Vibrational Properties of Bef2 at 300k
Narges Masoumi 1 , George Wolf 1 , Andrew Chizmeshya 1 Show Abstract
1 School of Molecular Sciences, Arizona State University, Tempe, Arizona, United States
We present a systematic DFT-based study of the compression behavior and stability of BeF2 polymorphs as a function of pressure along the 300K isotherm. Thermal corrections are computed using the theoretical phonon spectrum employing the quasi-harmonic approximation. The finite temperature compression equation of state obtained for a-quartz is compared with high-pressure experimental data up to 18 GPa, and with the results from previous static lattice calculations. The pressure dependence of the calculated phonon spectra will be used to elucidate the origin of possible dynamic instabilities, as suggested by observed anomalies in the Raman spectra at high pressure. The pressure dependent Raman spectra for a range of BeF2 polymorphs are obtained from first principles by calculating the activity of Γ-point phonon modes using derivatives of the dielectric tensor generated using a density functional perturbation theory approach. The resulting theoretical spectra are compared directly with in situ diamond anvil cell measurements of the pressure-dependent Raman spectra of the quartz and coesite phases. The calculated free-energies are used to study the thermodynamic stability of a variety of BeF2 polymorphs, including α-quartz, coesite, rutile-type and α-PbO2, and the role of thermal corrections is critically assessed vis a vis static lattice predictions for both the structure and compression behavior. Finally, the pressure dependence of BeF2’s properties are compared with those of other tetrahedral framework systems, notably SiO2 ,GeO2 and ZnCl2.
4:45 PM - CM1.2.07
Densification and Shear Mechanisms in the Plastic Deformation of Silcate Glasses
Shefford Baker 1 , Nicole Wiles 1 , Lisa Lamberson 1 Show Abstract
1 , Cornell University, Ithaca, New York, United States
It is well known that silicate glasses can deform extensively by plastic deformation in high pressure experiments and under point contacts in hardness and scratch tests. While this plastic deformation has been attributed to both shear (in normal glasses) and densification (in anomalous glasses), the atomic mechanisms involved in these deformation modes are poorly understood. We have used nanoindentation to study plastic deformation in a series of tectosilicate aluminosilicate glasses that were previously hydrostatically compressed to varying amounts, changing the relative amounts of shear and densification. Although mathematically it is possible to describe glass deformation as the sum of isochoric shear and pure dilatation components, we suggest that there are no actual deformation mechanisms available in silicate glasses than can produce shear without densification or densification without at least local shears. These results help to explain the complex plastic deformation modes in silicate glasses subjected to high pressures.
5:00 PM - *CM1.2.08
Giant Pressure Effect on Magnetism in Cubic Perovskite Sr1-xBaxCoO3 with Competing Magnetic Orders
Shintaro Ishiwata 1 2 Show Abstract
1 Department of Applied Physics, University of Tokyo, Tokyo Japan, 2 , JST-PRESTO, Kawaguchi, Saitama Japan
A class of perovskite oxides with unusually high valence 3d-transition metal ions such as Fe4+ and Co4+ has been of long-standing interest because of the unique magnetism inherent to the oxygen p hole character, exemplified by a novel helimagnetic order in a cubic perovskite SrFeO3 . It has been reported that the magnetic ground state of SrFeO3 is sensitive to the change in the lattice parameter as confirmed by the effect of the Ba substitution for Sr on the spin spiral . This experimental fact implies that this class of oxides is a fertile ground for novel magnetic phases inherent to competing magnetic interactions, which are highly dependent on the metal-oxygen bond length.
In this work, we have conducted Ba substitution for a cubic perovskite SrCoO3, which shows a room-temperature ferromagnetism , to study the effect of lattice expansion on magnetism. With the use of a high-pressure oxidization technique or a chemical oxidation technique, large single crystalline samples of Sr1-xBaxCoO3 with x up to 0.5 were successfully obtained. Based on magnetization measurements, we have established the magnetic phase diagram , in which the high-temperature ferromagnetic phase and a novel helimagnetic phase are competing at around x = 0.35. To discuss this surprising result from the viewpoint of the lattice change, we applied high pressure on the heilmagnetic compound with x = 0.4 and found that the ferromagnetic ground state is recovered by the high pressure as small as 0.7 GPa. This novel pressure effect can be regarded as a giant magnetovolume effect . Based on the transport measurements under pressure and first-principles calculations, the results will be discussed in terms of the competing magnetic order in the Co4+-O-Co4+ lattice.
 S. Ishiwata et al., Phys. Rev. B 84, 054427 (2011).
 N. Hayashi et al., Angew. Chem. Int. Ed. 50, 12547 (2011).
 Y. W. Long, et al., J. Phys.: Condens. Matter, 23, 245601 (2011).
 H. Sakai et al., in preparation
 S. Yokoyama et al., in preparation
Yue Chen, University of Hong Kong
Thomas Hammerschmidt, Ruhr-Universität Bochum
Alexey Kolmogorov, Binghamton University
Kuo Li, Center for High Pressure Science and Technology Advanced Research
Jung-Fu Lin, The University of Texas at Austin
CM1.3: Layered Materials and Method Developments
Wednesday AM, April 19, 2017
PCC North, 100 Level, Room 125 A
9:30 AM - *CM1.3.01
Giant Tunability of Thermal Conductivity in Multilayer MoS2 with Pressure
Yaguo Wang 1 , Xianghai Meng 1 , Tribhuwan Pandey 2 , Abhishek Singh 2 , Jung-Fu Lin 1 Show Abstract
1 , The University of Texas at Austin, Austin, Texas, United States, 2 Materials Research Centre, Indian Institute of Science, Bangalore India
Two-dimensional transition metal dichalcogenides (TMD) such as MoS2 are promising for applications in novel electronic and optical devices. One major drawback in the device applications of 2D materials, however, is related to their intrinsically anisotropic thermal conductivity in which cross-plane thermal conductivity is more than 10 times smaller than in-plane thermal conductivity. Consequently, cross-plane heat dissipation is not efficient, which has hindered the performance of the 2D devices. Strain has been found to be effective in tuning the band gap of the TMDs, but has yet been investigated in optimizing their thermal transport properties. With about 9% cross-plane compressive strain created by hydrostatic pressure in a diamond anvil cell, we observed about 12 times increase in the cross-plane thermal conductivity of multilayer MoS2. This drastic change arises from the greatly strengthened inter-layer interaction and heavily modified phonon dispersions along the cross-plane direction. The change in electronic thermal conductivity due to semiconductor to metal transition plays a minimal role. Our experimental and theoretical studies show that longitudinal acoustic phonons dominate the increase in cross-plane thermal conductivity under compressive strain; the saturation above 9% strain is associated with the combined effects from enhanced group velocity via the phonon hardening and reduced phonon lifetimes due to phonon unbundling. As a result, the anisotropic thermal conductivity in the multilayer MoS2 at ambient environment becomes almost isotropic under highly compressive strain, effectively transitioning from 2D to 3D heat dissipation. Our observation for the phonon-dominant change of thermal conductivity across the semiconductor-metal transition raises the prospective for designing electronic devices possessing both high electrical conductivity and high cross-plane thermal conductivity for effective heat dissipation, as well as heat modulators with controllable and directional heat flux. The concept for strain tuned 2D to 3D transition in the thermal transport property can be potentially extended to a larger ever increasing family of 2D van der Waals solids.
10:00 AM - CM1.3.02
Pressure and Composition Tuning of Optical Band Gap in Monolayer Transition Metal Dichalcogenides
Joonseok Kim 1 , Rafia Ahmad 2 , Tribhuwan Pandey 2 , Amritesh Rai 1 , Simin Feng 3 , Jing Yang 1 , Mauricio Terrones 3 , Sanjay Banerjee 1 , Abhishek Singh 2 , Deji Akinwande 1 , Jung-Fu Lin 1 Show Abstract
1 , The University of Texas at Austin, Austin, Texas, United States, 2 , Indian Institute of Science, Bangalore India, 3 , The Pennsylvania State University, University Park, Pennsylvania, United States
Since the first isolation of monolayer graphene, the atomically-thin layered structure of 2D materials gained plentiful interest. Various 2-dimensional (2D) materials, ranging from insulating hexagonal boron nitride (hBN), semiconducting transition metal dichalcogenides (TMDs) and phosphorene, and to semi-metallic graphene, are proposed as novel materials for future devices application, such as flexible, transparent, patch-like, and/or internet of things (IoT) devices. TMDs are composed of a sheet of transition metal atoms sandwiched between two layers of chalcogen atoms. Sulfides and selenides such as MoS2, MoSe2, WS2, and WSe2 are semiconductors with band gap ranging 1.2-2 eV. The indirect band gap in the bulk state increases with decreasing thickness and aligns directly in monolayer, dramatically enhancing optical behaviors. With various large-area monolayer synthesis methods developed, monolayer TMDs are proposed to be a candidate for all-around optoelectronic material.
Pressure/strain is an effective method to engineer properties of TMDs, similar to the silicon technology that has been utilizing strain to enhance the properties and boost the performance of devices. The band gaps of semiconducting TMDs are reported to decrease with increasing pressure and metallize at pressure range 20-30 GPa. In monolayer semiconducting TMDs on the other hand, optical band gaps increase with increasing pressure and reach direct to indirect band transition. Band gap of monolayer MoS2 was reported to increase from 1.85 eV to 2.07 eV and becomes indirect at 16 GPa. Optical band gap of WS2 was measured with pressure, and discovered that the band gap increase is more sensitive to pressure. The direct to indirect transition, inferred by vanishing photoluminescence, occurs at a pressure which is lower than that of MoS2.
In the case of Mo0.5W0.5S2, a ternary TMD with randomly dispersed Mo and W atoms, incident band gap, pressure dependence, as well as indirect transition pressure are in between the values of the two end members. It is expected that band structure could be modified by alloying the composition elements and applying pressure to TMDs. Interestingly at higher pressures, multi-layer MoWS2 alloy developed two additional undiscovered Raman peaks which are suggested to be inter-layer disorder-activated phonon modes. This implies that TMDs can develop inter-layer phonon modes if the inter-layer distance is compressed enough under pressure.
These pressure-composition dependent optoelectronic properties will allow us to precisely engineer the properties of TMD on demand. More complex 2-dimensional structures such as heterostructures, band edge engineering, and/or quantum well structure may also benefit from the universal tunability of band gaps.
10:15 AM - CM1.3.03
Atomistic Visualization of Layer-Tunnel Transition Pathways in MnO2
Yifei Yuan 1 2 , Bryan Byles 3 , Jun Lu 4 , Khalil Amine 4 , Ekaterina Pomerantseva 3 , Reza Shahbazian-Yassar 1 Show Abstract
1 , University of Illinois at Chicago, Chicago, Illinois, United States, 2 , Argonne National Laboratory, Argonne, Illinois, United States, 3 , Drexel University, Philadelphia, Pennsylvania, United States, 4 , Argonne National Laboratory, Argonne, Illinois, United States
MnO2 widely exists in the earth’s crust and marine sediment, and it is applied in catalysis, molecular sieves and energy storage. Based on how its structural units ([MnO6] octahedra) are linked, polymorphic MnO2 is categorized into two types: two-dimensional (2D) layered and one-dimensional (1D) tunnel phases. In general, layered MnO2 can transform into tunnel structures, such as σ-MnO2-α-MnO2 and buserite-todorokite phase evolution. Layer-tunnel transition happens naturally in marine sediments and exerts a strong influence on the concentration of micronutrient trace metals and marine chemistry. It can also be accelerated in lab synthesis hydrothermally, with the goal to obtain tunnels of controlled size. Therefore, layer-tunnel transition is important in marine chemistry, and also in selective tunnel synthesis for specific industrial and laboratory applications. Current mechanisms explaining the layer-tunnel transition include layer buckling-shearing, layer kinking-dissolution-recrystallization, and direct Mn migration from layer to the interlayer. Although the proposed mechanisms vary, one similarity is shared by implying the critical roles of intralayer [Mn3+O6] octahedron and its Jahn-Teller distortion in guiding the layer-tunnel transition, which is, however, poorly understood to date. Many questions are not answered, such as where the layer-tunnel transition nucleates, whether the transition is homogeneous or topotactic, and how various atomic species are spatially rearranged during the transition. Failing to answer these questions leads to poorly controlled synthesis of size-specific tunnels whose performance is thus degraded, and also results in difficulty to clarify the role of marine sediments in global trace element cycles.
Here, we study the layer-tunnel transition at sub-Å resolution using 10 Å-spaced layered buserite as the precursor subjected to varying hydrothermal conditions. The intermediate state from the layered buserite to tunnel todorokite is analyzed to track the atomic reconfiguration, compositional evolution and electronic structure change in this process. Layer-tunnel transition is found to proceed topotactically from the layer edge into the body, leading to the gradual splitting of 2D MnO2 layers into 1D nanowires. This transition requires multiple steps via formation of intermediate tunnels with large openings. The transition starts by macroscopic layer distortion from adjacent Jahn-Teller active [Mn3+O6] octahedra, followed by Mn3+ disproportionation reaction and layer-interlayer Mn migration, interlayer Mg2+ expulsion, and reconstruction of Mn-O bonds to build tunnel walls. The atomistic discovery of layer-tunnel transition can not only help to track the global trace element cycles and the evolution of the earth’s crust and marine sediments, but also guide the controlled synthesis and selective size-engineering of MnO2 tunnels for specific applications in catalysis, molecular sieves and energy storage.
10:30 AM - *CM1.3.04
High Strain Coupled Opto-Electro-Mechanics in Layered Materials
Joonseok Kim 1 , Jung-Fu Lin 1 , Deji Akinwande 1 Show Abstract
1 , The University of Texas at Austin, Austin, Texas, United States
Two-dimensional (2D) materials are solid crystals consisting of layered planes of atoms that results in anisotropic properties with strong contrast in in-plane and cross-plane physics. Under very high strain or pressure, the van der Waals gap reduces and the anisotropic differences diminishes leading to interesting physics including phase transition, electronic transition, structure transition, and thermal transitions. All in all, the behaviour of electrons, phonons and photons obey different symmetric rules under vertical pressure.
In this talk, we will discuss our research on high-pressure effects on monolayer and multilayer TMDs, black phosphorus and heterostructures and the subsequent dramatic consequences. We find the underlying physics is distinct when comparing monolayer to multilayer counterparts. In addition, performance can be tuned far beyond what is observed in conventional bulk solids. These performance parameters include bandgap, mobility, carrier concentration, etc.
Layered nanomaterials represent a new frontier where strain can play an effective role as a degree of freedom in controlling and tuning material structure and properties.
11:30 AM - *CM1.3.05
Strain-Induced Electronic Phase Changes in Layered Materials
Abhishek Singh 1 Show Abstract
1 , Indian Institute of Science, Bangalore India
Using first principles density functional theory (DFT), we investigated the effect of strain on phosphorene and transition metal dichalcogenides (TMDs). These materials show a universal phenomenon of reversible semiconductor-metal (S- M) transition under various types of strain. This modification of electronic band structure can significantly affect the electronic transport as well as optoelectronic properties. We have also observed robust dynamical stability over a wide strain range irrespective of the type of applied strain. Furthermore, strain has been shown to induce fundamental properties changes in thermoelectric materials. This inherent ease of tunability of electronic properties of these materials is expected to pave way for further fundamental research leading to multi- physics devices.
12:00 PM -
12:15 PM - CM1.3.07
A Transition in the Nature of the Electron Transport that Occurs within Wurtzite Zinc Oxide in Response to the Application of Stress And the Concomitant Potential for Electron Device Applications
Poppy Siddiqua 1 , Michael Shur 2 , Stephen O'Leary 1 Show Abstract
1 School of Engineering, The University of British Columbia, Kelowna, British Columbia, Canada, 2 ECSE, Rensselaer Polytechnic Institute, Troy, New York, United States
The steady-state electron transport characteristics of bulk wurtzite ZnO are found to be a strong function of the non-parabolicity coefficient associated with the lowest energy conduction band valley. Hadi et al.  suggested that the non-parabolicity in ZnO could be adjusted by varying the growth conditions and optimized for electron device applications. Stress resulting, for example, from pseudomorphic growth on a heterogeneous substrate and due to differences in the thermal expansion coefficients between the substrate and the epitaxial materials, has the potential to change the band structure in a rather dramatic way and this will have an impact on the resultant device performance. For example, the observed enhancements in the electron and hole mobilities associated with strained SiGe layers that have been shown to be directly related to the changes in the band structure that accompany the application of stress . Since the stress influences the electron effective mass and the inter-valley energy separations, it should also affect the non-parabolicity coefficient. Thus, we expect that the application of stress will result in variations in the electron transport characteristics. Within the scope of this paper, we will explore this dependence and examine the potential for electron device applications and conclude that the sensitivity of the electron transport characteristics associated with bulk wurtzite ZnO to variations in the non-parabolicity coefficient of the lowest energy conduction band valley adds a new dimension to band structural engineering design considerations that offers opportunities for enhanced electron device performance.
 W. A. Hadi, S. K. O’Leary, M. S. Shur, and L. F. Eastman, “The sensitivity of the steady-state electron transport within bulk wurtzite zinc oxide to variations in the non-parabolicity coefficient,” Solid State Communications, Volume 151, pages 874-878, 2011.
 M.V. Fischetti, S.E. Laux, “Band structure, deformation potentials, and carrier mobility in strained Si, Ge, and SiGe alloys,” Journal of Applied Physics, Volume 80, pages 2234-2252, 1996.
 M. Dyakonov, M.S. Shur, “Consequences of space dependence of effective mass in heterostructures, Journal of Applied Physics, Volume 84, pages 3726-3730, 1998.
12:30 PM - CM1.3.08
Liquid-Liquid Transition in Ti
Byeongchan Lee 1 , Geun Woo Lee 2 Show Abstract
1 , Kyung Hee University, Yongin Korea (the Republic of), 2 , Korea Research Institute of Standards and Science, Daejon Korea (the Republic of)
Materials in an amorphous structure, either solid or liquid, are known to have an ordering. Even liquid transition metals may be identified from one another as each liquid transition metal has a short-range order (SRO) that can be substantially different than that of other liquid metals.
Recently, some elemental materials, e.g. Si and Ce, are found to show phase transformations even in an amorphous state, and the condition necessary for such polyamorphism is attracting attention in the absence of the effects from alloying as in metallic glasses. Elemental materials known to show polyamorphism have two things in common. First, they are either sp-valent or f-valent materials. Second, they have a precursor in the melting slope, i.e. a negative melting slope or the maximum in the melting slope.
In this talk, the liquid-liquid transition of Ti is presented. This is the first system found in an elemental form to show a phase transition with a positive melting curve. The transition is induced by pressure, and determined from the SRO change with first principles molecular dynamics calculations. The transition pressure is around 40 GPa, and we have found a substantial change in SRO with the growing number of short bonds. The driving force of the liquid-liquid transition is found to be electronic-structure effects. In addition, the local order change of liquid Ti under pressure is compared and contrasted with that of liquid Ni. Liquid Ni shows no SRO change, and we believe that the difference between liquid Ti and liquid Ni is applied in general to early transition metals versus late transition metals.
The results extend the range and generalize the necessary condition of polyamorphism to d-valent elemental materials along with the positive melting slope. We conclude that the polyamorphisim is not necessarily associated with specific bands, but rather determined by the symmetry of electronic structures of constituent elements.
12:45 PM - CM1.3.09
Quasi-Phase Transition of Single File Water Molecules
Xuedan Ma 1 , Sofie Cambre 2 , Wim Wenseleers 2 , Stephen Doorn 3 , Han Htoon 3 Show Abstract
1 , Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois, United States, 2 Experimental Condensed Matter Physics Laboratory, University of Antwerp, Antwerp Belgium, 3 Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Single-walled carbon nanotubes (SWCNTs) filled with water has attracted great attention, mainly because of their analogy with biological membranes and potential applications in nanofluidics technologies. For SWCNTs with diameters d larger than 0.9 nm, confined water can transform into low-dimensional ice-nanotubes, with the freezing point being critically dependent on the SWCNT diameters. Even more intriguing is the behavior of water in small (d < 0.9 nm) and yet fillable SWCNTs where mutual passage of molecules is excluded and water molecules form one-dimensional single-file chains. Although the restricted lateral mobility in a single file precludes distinguishing liquid from solid phases, a transition in the orientational order of the water molecules cannot be excluded, for which experimental evidence is still lacking.
Optical properties of SWCNTs are extremely sensitive to the presence of encapsulated molecules. In this study, we present measurements of changes in the photoluminescence (PL) spectra of water-filled and empty (6,5) SWCNTs subjected to temperature changes from room temperature to 4.2 K. With the PL spectra of the empty SWCNTs as the reference, we are able to determine the spectral shifts solely induced by the confined water molecules. We observe that superimposed on the linear shifts of the PL spectra of the empty SWCNTs, the spectra of the water-filled SWCNTs show a significant transition at about 150 K. Our molecular dynamics simulations indicate that this transition is caused by changes in the dipole orientations of the encapsulated water molecules. These findings provide new insights in the phase behavior of confined single file water molecules, and demonstrate PL spectroscopy as a sensitive tool in detecting such phase transitions.
CM1.4: Phase Transition Under Pressure
Wednesday PM, April 19, 2017
PCC North, 100 Level, Room 125 A
2:30 PM - *CM1.4.01
Single Crystal Structural Studies at Multimegabar Pressures and High Temperatures
Leonid Dubrovinsky 1 , Natalia Dubrovinskaia 1 Show Abstract
1 , University of Bayreuth, Bayreuth Germany
Laser heating techniques in diamond anvil cells (DACs) cover a wide pressure-temperature range – above 300 GPa and up to 5000 K. Recent advantages in on-line laser heating techniques resulted in a significant improvement of reliability of in situ X-ray powder diffraction studies in laser-heated DACs, which have become routine at a number of synchrotron facilities including specialized beam-lines at the 3rd generation synchrotrons. However, until recently, existing DAC laser-heating systems could not be used for structural X-ray diffraction studies aimed at structural refinements, i.e. measuring of the diffraction intensities, and not only at determining of lattice parameters. The reason is that in existing DAC laser-heating facilities the laser beam enters the cell at a fixed angle, and a partial rotation of the DAC, as required in monochromatic structural X-ray diffraction experiments, results in a loss of the target crystal and may be even dangerous if the powerful laser light starts to scatter in arbitrary directions by the diamond anvils. In order to overcome this problem we have develop a portable laser heating system and implement it at diffraction beam lines (ID9 and ID27 at ESRF, and P02 at PETRA III).
We demonstrate the application of this system for simultaneous high-pressure and high-temperature powder and single crystal diffraction studies using examples of studies of chemical and phase relations in the Fe-O system, transition metals carbonates, and silicate perovskites.
3:00 PM - CM1.4.02
Strain Engineered Pyrochlore at High Pressure
Dylan Rittman 1 , Katlyn Turner 1 , Sulgiye Park 1 , Antonio Fuentes 2 , Changyong Park 3 , Rodney Ewing 1 , Wendy Mao 1 Show Abstract
1 , Stanford University, Stanford, California, United States, 2 , Cinvestav Unidad Saltillo, Saltillo Mexico, 3 , Carnegie Institution of Washington, Argonne, Illinois, United States
Strain engineering is a promising method for next-generation materials processing techniques. Here, we use mechanical milling and annealing followed by compression in diamond anvil cell to tailor both the intrinsic and extrinsic strain experienced by technologically important pyrochlore oxides, Dy2Ti2O7 and Dy2Zr2O7. Raman spectroscopy, X-ray pair distribution function analysis, and X-ray diffraction were utilized to characterize atomic order over a wide range of spatial scales at ambient conditions, providing a comprehensive picture for understanding the high-pressure structural response. Raman spectroscopy and X-ray diffraction were further used to interrogate the material in situ at high pressure. The high-pressure structural behavior is found to be dependent on the sample’s initial atomic order, defined by its crystal structure, concentration of cation anti-site and anion Frenkel defects, and internal strain. Overall, it is shown that properly tuning a material’s initial defect profile can lower the critical pressure of the phase transformation, and enhance the transformation’s kinetics, without sacrificing significant mechanical integrity.
3:15 PM - CM1.4.03
Jahn-Teller Phase Transition of Icosahedral Units Under Pressure
Seyed Hossein Nasrollahi 1 , Dimitri Vvedensky 1 Show Abstract
1 , Imperial College London, London United Kingdom
Non-linear molecules undergo distortions when the orbital degeneracy of the highest occupied level is lifted by the Jahn-Teller effect. If such molecules or clusters of atoms are coupled to one another, the system may experience a cooperative Jahn-Teller effect (CJTE). In this work, we describe a model of how the CJTE leads to the crystallization of the disordered phase. The model Hamiltonian is based on a normal mode decomposition of the clusters in order to maintain the symmetry labels. We take account of the electron-strain and the electron-phonon couplings as well as an external pressure imposed on the system. By displacing the coordinates of the oscillators, we obtain a term that explicitly couples the Jahn-Teller centers, enabling us to perform a mean-field analysis. The calculation of the free energy then becomes straightforward, and obtaining pressure-temperature phase diagrams in various regimes follows from the minimization of this free energy. Taken together, these results may serve as a paradigm for crystallization near the transition temperature, where the atoms tend to form clusters of icosahedral symmetry.
4:30 PM - CM1.4.05
Graphenoid Carbons—Systematic Search, Properties and Perspectives
Stefano Leoni 1 , Samuel Jobbins 1 Show Abstract
1 , Cardiff University, Cardiff United Kingdom
Research on novel carbon allotropes never loses momentum. The expectation of a larger variety than what is known so far is motivated by its ability to form tetrahedral sp3 compounds like diamond, as well as planar sp2 graphite. Between those two limiting geometries a manifold of intermediate cases is known, from fullerenes to nanotubes, from hyperbolic Schwarzites to carbon foams. While the planar sp2 unit implies layers of merged six-rings, as regular triangles tile the Euclidean plane, this is not necessarily true for structures based on sp3 centres. Diamond consists of adamantane-like cages of fused six-rings in chair conformation. On the other hand 3D structures with planar six-rings and tetrahedral centres seem to be intrinsically a problem, unless there is a certain amount of odd rings in the structure. Combination of odd and even rings is key to hard and transparent carbon materials, while entangling 1D nanotubes in regular pattern produces superior hydrogen storage materials. Dirac fermions appears in single sheet graphene, while electronic properties can be further tuned in few-layers gaphites. In the attempt to expand the catalogue of carbons, we have found novel graphenoid strcutures, based on sp2 centers, which nonetheless are not planar, i.e. they represent 3D gaphenes. They define a novel class of compounds, very poorly investigated so far, that can be derived from generalised graphite compounds, whereby different tilings of the plane can be used as means for their classification. Along this way, different graphites as well as graphenoid carbons can be obtained, which hosts interesting electronic properties. The chirality of some of them may represent the starting point towards pure carbon 3D Weyl fermion materials, which so far could be realised in exotic materials only. The possibility of experimentally accessing these materials from suitable precursors can be studied by accurate mechanistic investigation based on advanced molecular dynamics techniques, showind details of pressure-induced material formation.
4:45 PM - CM1.4.06
Aliphatic Layered System with Emergent Chain Melting: Phase Transition Induced by Material Segmentation
Zichao Ye 1 , Leslie Allen 1 Show Abstract
1 , University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Aliphatic chain melting of biological membranes depends on the chain length and the nature of the hydrophilic head segments. Meanwhile, size miniaturization of nanoscale materials highlights the significance of certain discrete structural regions (e.g. surface, interface) that are negligible in bulk materials. As a model material for aliphatic chain melting, size-dependent phase transition of silver alkanethiolate (AgSCnH2n+1 or AgSCn, n=1-16) is measured and is employed to model the local structures of polymethylene systems. A novel synthesis method, which generates highly-ordered aliphatic layers, is coupled with NanoDSC, a highly-sensitive technique developed in our group, to reveal the emergent chain melting. (1) Odd/even melting appears only when interlayer interfaces are formed, whereas it is absent in single layer melting; (2) melting point and enthalpy unexpectedly increase as chain length decreases to n≤5. The mechanism of the emergent melting behavior is studied in terms of local chemical environment using 13C NMR. The alkyl chain is modeled and divided into three segments: (i) the normal mid-chain region contains n-5 CH2 groups with similar environment and identical melting enthalpy; (ii) 3 CH2 units closest to chain tail are grouped together as they are the only groups that show parity-dependent chemical environment and is responsible for the odd/even aliphatic melting; (iii) the head segment contains the remaining 2 CH2 groups closest to the high-polarity Ag-S slab and possesses the most positive charge density. The melting of the mid-chain segment determines the homogeneous chain melting widely exhibited in polyethylene and paraffin wax. The unique short chain melting (n≤5) is caused by the disappearance of the mid-chain region and the merge of the discrete head and tail segments. Layers grown at such small scales force us to deal with the reality that different segments do not have mathematically planar boundaries but have an intrinsic thickness. The overlap of these segments changes the nature of the system from bulk to discrete properties, and thus produces new material behaviors. Chain melting of other aliphatic systems can be predicted by the methodology in this work, and be manipulated by tailoring the nature of the discrete segments either chemically (e.g. introduce unique groups into alkyl chains) or environmentally (e.g. introduce ions to the surroundings of biological membranes).
5:00 PM - *CM1.4.07
Adventures with 5d Orbitals at High Pressure
Daniel Haskel 1 Show Abstract
1 X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois, United States
While first-row (3d) transition metal (TM) oxides continue to provide a rich playground for studies of electron correlations, recent focus has shifted to third-row (5d) TM oxides in the search for novel quantum states. The sizable spin-orbit interaction in heavy 5d ions, coupled with reduced on-site Coulomb interactions as a result of the large spatial extent of 5d orbitals, create unique experimental and theoretical opportunities for discovery of new electronic phases of matter. We have studied some of the consequences of enhanced S-O coupling and spatial extent of 5d orbitals on the electronic structure and magnetic (exchange) interactions in a novel “iridate” magnetic insulator, Sr2IrO4. The high-brilliance, penetration power, and polarization/energy tunability of synchrotron radiation enable the use of x-ray absorption spectroscopy (including circular dichroism) and resonant magnetic scattering techniques in the diamond anvil cell providing exquisite sensitivity to the evolution of electronic correlations at high pressure. Among other findings, we discovered that pressure leads to frustration of exchange interactions in the square lattice of entangled spin-orbital Iridium moments and emergence of quantum paramagnetism, possibly a quantum spin liquid phase. Higher brilliance, 4th generation synchrotron light sources now in development around the globe will bring unprecedented opportunities for studies of electronic/magnetic order at the limit of static high-pressure generation technologies.
Work at Argonne is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC-02-06CH11357.
CM1.5: Poster Session
Wednesday PM, April 19, 2017
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - CM1.5.01
Micro-Strain Dominant Solid-Solid Phase Transition under High Pressure
Qiwei Hu 1 , Li Lei 1 , Duanwei He 1 Show Abstract
1 , Sichuan University, Sichuan China
Nearly all classes of materials show solid-state phase transitions. Understanding transformations from one crystal to another is a key topic in materials science because in many cases they determine material properties. While observing solid–solid phase transitions in-situ is a great challenge and is often only possible via computer simulations or in model systems. Recently, a study of polymeric colloidal particles, where the particles mimic atoms, revealed an intermediate state in the transition from one solid to another. However, the reason underlying the solid-solid transition remains open. Here we present experimental evidences for micro-strain dominance of a solid–solid transition in a ‘real’ atomic system in LiAlO2 and LiGaO2.During hydrostatic compression, γ-LiAlO2 (tetrahedral, space group P41212) transforms to δ-LiAlO2 (tetrahedral, space group I41/amd) at about 4GPa. In the coexisting region, there is enormous microscopic strain stemming from the lattice mismatch. The nonisotropic compression in the crystal is strong enough to overcome this strain and realizes the transition without intermediate atomic state.During hydrostatic compression β-LiGaO2 transforms to γ-LiGaO2 . However, the (110) refection exhibits abnormal behavor before the phase transition. It can be infered that β-LiGaO2 transforms to γ-LiGaO2 through a intermediate state in the reason that the strain from β-LiGaO2 to γ-LiGaO2 is too great to get through. The strain is as ten times as the strain in LiAlO2 .We conclude that the mechanism of high-pressure solid-solid transition is depended on the microscopic strain stemming from the lattice mismatch. It will pass through an intermediate state when the strain is too strong.
9:00 PM - CM1.5.02
Spectroscopic Studies of the Effect of High Pressure on Poly[(R)-3-Hydroxybutyrate-Co-(R)-3-Hydoxy-Hexanoate] (PHBHx) Random Copolymers Using a Diamond Anvil Cell
Chinmay Pawar 1 , Brian Sobieski 1 , Changhao Liu 1 , Liang Gong 1 , Isao Noda 1 , Bruce Chase 1 , John Rabolt 1 Show Abstract
1 Materials Science and Engineering, University of Delaware, Newark, Delaware, United States
Poly[(R)-3-hydroxybutyrate-co-(R)-3-hydoxyhexanoate] (PHBHx) is a random copolymer, which is biosynthesized and enzymatically degrades into CO2 and H2O. It is a polymorphic material that exists at room temperature and atmospheric pressure, in its stable α form with a helical backbone conformation. However, under certain processing conditions it can assume the metastable β crystalline form, with the polymer backbone adopting a planar zig zag conformation1. We have been exploring various processing protocols in order to increase the β content and possibly “lock-in” this planar zig zag conformation, which would greatly enhance the mechanical properties of PHBHx. One approach was to investigate the effect of high pressure on the conformation and crystal structure of PHBHx. A calibrated diamond anvil cell was used to investigate any conformational transitions using FTIR.
Different molar compositions of PHBHx were put under high pressure (on the order of a few GPa) and observed with FT-IR spectroscopy. Our initial observations indicate that the effect of pressure causes significant frequency shifts (10-15 cm-1) and intensity changes, which may possibly be attributed to a phase transition. The effect of high pressure on PHBHx as a function of the molar composition of Hx will also be presented.
1 Investigation of β-form Crystal Structure in Electrospun Poly [(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] (PHBHx) Nanofibers: From Fiber Meshes to Single Fibers,
Liang Gong, D. Bruce Chase, Isao Noda, Jinglin Liu, David C. Martin, Chaoying Ni, C. J. McBrin and John F. Rabolt, Macromolecules 2015; 48(17), 6197-6205.
9:00 PM - CM1.5.03
Orbital Ordering in Certain Vanadium Spinel Oxides—First-Principle Electronic and Phonon Based Approach
Dibyendu Dey 1 , T. Maitra 2 , A. Taraphder 1 3 Show Abstract
1 Department of Physics, Indian Institute of Technology, Kharagpur, Kharagpur, West Bengal, India, 2 Department of Physics, Indian Institute of Technology, Roorkee, Roorkee, Uttarakhand, India, 3 Centre for Theoretical Studies, Indian Institute of Technology, Kharagpur, Kharagpur, West Bengal, India
In Vanadium spinels intricate couplings between various degrees of freedom like magnetic, orbital, charge and lattice, make the physics of these systems extremely interesting. In most of these systems, multiple phase transitions such as structural and magnetic ones are observed with temperature and the ground state is often found to have both magnetic and orbital ordering. MnV2O4 and FeV2O4 are two such spinel oxides where ground state orbital ordering is under continuous debate. In view of the recent debate on the predicted orbital ordering in Mn doped FeV2O4 from the experimental observations on the system, we have investigated the compound Fe1−xMnxV2O4 using first principles density functional theory calculations as a function of doping (x). The effects of Coulomb correlation and spin-orbit interaction within GGA+U and GGA+U+SO approximations are incorporated in our calculations. We have observed that for x≤0.6, the orbital order at V sites consists of a linear superposition of dxz and dyz orbitals of the type dxz ± dyz. Whereas for x>0.6, it is A-type ordering where one 3d electron occupies dxy orbital at every V site and another electron occupies either dxz or dyz orbital alternatively along c axis. On the other hand at Fe sites, ferro-orbital ordering of dx2−y2 orbitals is observed for x≤0.6 and of dz2 orbitals for x>0.6. Effect of spin-orbit interaction is found to be insignificant in the entire range of doping indicating non-involvement of complex orbitals in the ordering process.
In addition, ab-initio phonon calculations within density functional theory framework have been done to obtain the nature of orbital ordering in the low temperature structure of MnV2O4 from the analysis of corresponding Raman spectrum. Orbitally active V3+ ions contribute to the Raman spectrum through the d-d excitations between neighbouring V ions. A detailed analysis of Raman spectra established the nature of orbital ordering at V sites to be A-type antiferro-orbital ordering. Stability and the orbital ordering of this compound are found to be correlation driven.
Yue Chen, University of Hong Kong
Thomas Hammerschmidt, Ruhr-Universität Bochum
Alexey Kolmogorov, Binghamton University
Kuo Li, Center for High Pressure Science and Technology Advanced Research
Jung-Fu Lin, The University of Texas at Austin
CM1.6: Properties of Materials Under Pressure
Thursday AM, April 20, 2017
PCC North, 100 Level, Room 125 A
9:00 AM - *CM1.6.01
Novel High-Pressure Phenomena Discovered through Crystal Structure Prediction
Artem Oganov 1 2 Show Abstract
1 , Skolkovo Institute of Science and Technology, Moscow Russian Federation, 2 , Stony Brook University, Stony Brook, New York, United States
Recent methods of crystal structure prediction have opened wide opportunities for exploring materials at extreme conditions and perform computational screening for materials with optimal properties for various applications. In my laboratory, we have developed a very powerful evolutionary algorithm USPEX [1,2], enabling prediction of both the stable compounds and their crystal structures at arbitrary conditions, given just the set of chemical elements. Recent developments include major increase of efficiency and extensions to low-dimensional systems and molecular crystals  (which allowed large structures to be handled easily, e.g. Mg(BH4)2  and H2O-H2 ) and a new technique called evolutionary metadynamics .
Some of the results that I will discuss include:
1. Theoretical and experimental evidence for a new partially ionic phase of boron, γ-B  and an insulating and optically transparent form of sodium .
2. Predicted stability of “impossible” chemical compounds that become stable under pressure – e.g. Na3Cl, Na2Cl, Na3Cl2, NaCl3, NaCl7 , Mg3O2 and MgO2 .
3. Novel compounds in the C-H-O, N-H and Mg-Si-O systems, and implications for interiors of giant planets.
4. Novel compounds of helium, stable at experimentally reachable pressures, Na2He and Na2HeO.
 Oganov A.R. et al, J.Chem.Phys. 124, 244704 (2006).
 Lyakhov A.O. et al., Comp. Phys. Comm. 184, 1172-1182 (2013).
 Zhu Q. et al, Acta Cryst. B68, 215-226 (2012).
 Zhou X.-F. et al, Phys. Rev. Lett. 109, 245503 (2012).
 Qian G.R. et al. Sci.Rep. 4, 5606 (2014).
 Zhu Q. et al, Cryst.Eng.Comm. 14, 3596-3601 (2012).
 Oganov A.R. et al, Nature 457, 863 (2009).
 Ma Y. et al, Nature 458, 182 (2009).
 Zhang W.W. et al, Science 342, 1502-1505 (2013).
 Zhu Q. et al., Phys. Chem. Chem. Phys. 15, 7796-7700 (2013).
9:30 AM - CM1.6.02
Magnetism of Transition Metals under High Pressure—Combined DFT and K-XMCD Study
Yaroslav Kvashnin 1 , Raffaella Torchio 2 , Sakura Pascarelli 2 Show Abstract
1 , Uppsala University, Uppsala Sweden, 2 , European Synchrotron Radiation Facility (ESRF), Grenoble France
Thanks to the development of the diamond anvil cells (DACs), it is now possible to reach the pressures exceeding that in the Earth's core.
In our work, we investigate the behaviour of transition metals under such extreme conditions and look at the pressure-driven phase transitions. We use synchrotron-radiation-based techniques such as x-ray diffraction and K-edge x-ray magnetic circular dichroism (K-XMCD) in order to obtain both structural and magnetic information about the systems under consideration. First-principles calculations, based on density functional theory (DFT) are employed to interpret the experimental data.
In this talk I will show the results for elemental Co, Ni and FeCo alloy, which we obtained with this combined approach, concentrating mostly on the theoretical part of the work.
First, we have demonstrated that nickel remains magnetic even under pressures exceeding 200 GPa . The DFT calculations predict a different pressure dependence of orbital and spin moments, which is also reflected in K-XMCD data. We argue that this behaviour is general for transition metals and explain its origin.
Cobalt undergoes a transition from the ferromagnetic hcp phase to the nonmagnetic fcc one around 100 GPa . We suggest that suggest that there are few more transitions taking place at lower applied pressure (~80 GPa), which are of Lifshitz type . It is found that these Lifshitz transitions are responsible for the anomalies in various elastic properties, observed experimentally, and potentially lead to the stabilisation of a noncollinear spin arrangement in highly compressed hcp phase.
Finally, we studied equiatomic FeCo alloy, which exhibits a bcc-hcp transition at around 30-40 GPa, depending on the degree of chemical order. Interestingly, the K-XMCD signal measured on both Fe and Co edges dissapears above the structural transformation. Ab initio calculations suggest that it can be related with the emergence of antiferromagnetic order among Fe spins .
 R.Torchio, et al., PRL 107, 237202 (2011)
 R.Torchio, et al., PRB 94, 024429 (2016)
 YK, W.Sun, I. Di Marco, O. Eriksson, PRB 92, 134422 (2015)
 R.Torchio, et al., PRB 88, 184412 (2013)
9:45 AM - CM1.6.03
Enhanced Properties of Organic-Inorganic Halide Perovskites via High Pressure Treatments
Xujie Lu 1 , Yonggang Wang 2 , Konstantinos Stoumpos 3 , Qingyang Hu 4 , Wenge Yang 5 , Hongwu Xu 1 , Mercouri Kanatzidis 3 , Quanxi Jia 1 Show Abstract
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 , University of Nevada Las Vegas, Las Vegas, Nevada, United States, 3 , Northwestern University, Evanston, Illinois, United States, 4 , Carnegie Institution of Washington, Washington, District of Columbia, United States, 5 , Center for High Pressure Science and Technology Advanced Research, Shanghai China
Organic-inorganic halide perovskites have demonstrated great potential for photovoltaics with high power conversion efficiency over 22% and low processing cost. However, their low stability results in degradation of the photovoltaic devices, which raises a question whether the high efficiency and outstanding stability can be realized concurrently. Using state-of-the-art high-pressure techniques coupled with in situ synchrotron and property measurements, here we report a comparative investigation on the structural, electrical, and optoelectronic properties of the halide perovskites under high pressures. Our results show great improvements in structural stability, electrical conductivity and visible-light responsiveness after high-pressure treatments. This study demonstrates that high pressure can be an effective approach for tuning the structure and properties of organic-inorganic hybrid perovskites.
10:00 AM - CM1.6.04
Pressure Induced Polymerization and Enhanced Conductivity of Metal Acetylides
Kuo Li 1 , Haiyan Zheng 1 , Lijuan Wang 1 , Yajie Wang 1 , Xiao Dong 1 Show Abstract
1 , Center for High Pressure Science and Technology Advanced Research, Beijing China
Pressure induced polymerization of metal acetylide was predicted in many papers and was evidenced in our previous reports as well as other investigations. Three phases of calcium acetylide were reported under ambient pressure and we found the indirect evidences of phase VI under high pressure as well as evidences of fragment of calcium polyacence by in situ neutron diffraction experiment and several other techniques including GC-MS and IR. This pressure induced polymerization process is accompanied with 107 fold enhancement of electrical conductivity. For a further step, by changing the experimental conditions, we observed the crystallized calcium polyacetylide and calcium polyacene. We also studied the pressure induced polymerization of Li2C2. Similarly, we found a 109 fold enhancement of conductivity when compressing Li2C2 to ~40 GPa, which is irreversible when recovered to ambient pressure. Phase transitions from the Immm phase to Pnma phase was identified, which is an insulator-metal phase transition and partly responsible for the enhancement of conductivity. When compressed to higher pressure, the C22- anions in Li2C2 polymerize into ribbon structures, which is recoginzed in in situ IR spectra. When decopressed, the polymerized carbon anions transform into C3 anions and other carbon species, which is probably a disproporation and likely lead to the decomposition of Li2C2. Our research uncovered the diversities of the chemcial reactions in metal acetylides under external pressure and provided new method to synthesize functional metal carbide conductors.
10:15 AM - CM1.6.05
High Pressure for Parameterization of the Proton-Phonon Coupling in Solid Electrolytes
Artur Braun 1 , Qianli Chen 1 2 3 Show Abstract
1 , Empa-Swiss Federal Laboratories for Materials Science and Technology, Duebendorf Switzerland, 2 , Shanghai Jiao Tong University, Shanghai China, 3 Physics Department, ETH Zürich, Zürich Switzerland
Protons in solids are not always welcome. They can cause brittleness in pressure vessels. They can make electric shorts in semiconductors. They can promote dielectric breakdown in insulators. Because of the abundance of humidity (H2O) in ambient environment and the crystalline imperfections (defects) in materials, water molecules like to enter some materials and settle as hydrates or hydrides structures. Oxygen and protons become part of the structure. Upon thermal excitation, the hydroxyl bonds may become hydrogen bonds which eventually " melt ". The protons may then liberate and become electric charge carriers, which lend them a particular new function in solid electrolytes as proton conductors. We have in the last couple of years observed and investigated the biography and lifestyle of such protons from localization to de-localization. The proton is an elusive player and not always easy to make out. With a combination of neutron and synchrotron based scattering and spectroscopy methods, along with electroanalytical techniques, we have increased our understanding of the proton dynamics and its structural origin, which is important for super-protonic conductivity. We have investigated the oxygen vacancy filling of engineered oxygen deficient BCY by water molecules with impedance spectroscopy and ambient pressure XPS , which enabled us to sketch a detailed picture of the correlation of molecular and electronic structure changes, with concomitant onset of proton conductivity at higher temperatures. We thus could design experiments, where the proton-phonon coupling was quantitatively investigated with high pressure and high temperature impedance spectroscopy combined with quasi-elastic neutron scattering [2,3]. Supported by pressure dependent XRD and Raman scattering data [4,5] we correlated the proton jumping parameters with the temperature and found that the proton jump times follow a polaron relation [6,7].
 Q. Chen et al., Chem. Mater. 25 (23), 4690 (2013).
 Q. Chen et al., Solid State Ionics 252, 2 (2013).
 Q. Chen et al., High Pressure Research 32(4), 471 (2012).
 Q. Chen et al., J. Phys. Chem. C 115 (48), 24021 (2011).
 Q. Chen et al., J. Eur. Ceram. Soc. 31 (14), 2657 (2011).
 Q. Chen et al., Appl. Phys. Lett. 97, 041902 (2010)
 A. Braun et al., Appl. Phys. Lett., 95, 224103 (2009).
10:30 AM - *CM1.6.06
High Pressure Synthesis of Polymorphic Phase of Boron Nitride—2D Wide Bandgap Nature and Superhard Materials
Takashi Taniguchi 1 Show Abstract
1 , National Institute for Materials Science, Tsukuba Japan
Hexagonal boron nitride BN (hBN) and cubic BN (cBN) are known as the representative crystal structures of BN. The former is chemically and thermally stable, and has been widely used as an electrical insulator and heat-resistant materials. The latter, which is a high-density phase, is an ultra-hard material second only to diamond. In addition to those, wruztite BN (wBN) is also known as other polymorphic phase. As crystal growth technique is not, however, applicable for wBN due to its thermodynamically metastable nature, fundamental properties of wBN with bulk crystalline form is not well studied so far.
Among those BN crystals, some progresses in the synthesis of high purity BN crystals were achieved by using Ba-BN as a growth solvent material at high pressure (HP) of 5.5GPa. Band-edge natures (cBN Eg=6.2eV and hBN Eg=6.4eV) were characterized by their optical properties. The key issue to obtain high purity crystals is to reduce oxygen and carbon contamination in the HP growth circumstances. Then an attractive potential of hBN as a deep ultraviolet (DUV) light emitter and also superior properties as substrate of graphene devices were realized. By using high purity hBN crystal as a starting materials, high purity cBN sintered body and also highly oriented wBN crystalline form were obtained by high pressure phase transformation process.
In this paper, recent studies on BN polymorphic phases obtained at high pressure with respect to impurity control and their function will be reported.
11:30 AM - *CM1.6.07
Nanocrystalline Diamond—Unique Carbon Material for Ultra-High Pressure Generation and Optical Applications
Natalia Dubrovinskaia 1 , Leonid Dubrovinsky 1 Show Abstract
1 , University of Bayreuth, Bayreuth Germany
In high pressure (HP) research, an achievable static pressure limit is imposed by available strong materials and design of HP devices. Achieving higher and higher pressures will open new horizons for a deeper understanding of matter and the discovery of new physical and chemical phenomena at extreme conditions. Recently we developed a new technique of ultra-high static pressure generation in a double-stage diamond anvil cell. Nanocrystalline diamond (NCD) balls synthesized from glassy carbon were used as secondary anvils. Due to unique properties of NCD, this technique allowed us to reach pressures beyond 1 TPa and to study the behavior of a number of materials at such extreme conditions. In this contribution we will report on unique properties of the NCD material and its applications.
12:00 PM - CM1.6.08
Order Parameter Aided Efficient Phase Space Exploration under Extreme Conditions
Amit Samanta 1 , Eric Schwegler 1 , Jonathan Belof 1 , Alexander Chernov 1 Show Abstract
1 , Lawrence Livermore National Laboratory, Livermore, California, United States
Physical processes in nature exhibit disparate time-scales, for example time scales associated with processes like phase transitions, various manifestations of creep, sintering of particles etc. are often much higher than time the system spends in the metastable states. The transition times associated with such events are also orders of magnitude higher than time-scales associated with vibration of atoms. Thus, an atomistic simulation of such transition events is a challenging task. Consequently, efficient exploration of configuration space and identification of metastable structures in condensed phase systems is challenging. In this talk I will illustrate how we can define a set of coarse-grained variables or order parameters and use these to systematically and efficiently steer a system containing thousands or millions of atoms over different parts of the configuration. This order parameter aided sampling can be used to identify metastable states, transition pathways and understand the mechanistic details of complex transition processes. I will illustrate how this sampling scheme can be used to study phase transition pathways and phase boundaries in prototypical materials, like SiO2, Ge under high-pressure conditions.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
12:15 PM - CM1.6.09
Molecular Dynamics Studies of the Melting of Copper with Edge and Screw Dislocations at High Pressures
Clarence Matthai 1 , Jessica Rainbow 1 Show Abstract
1 , Cardiff University, Cardiff Wales United Kingdom
Molecular dynamics simulations of the melting process of bulk copper were
performed using Large-scale Atomic/Molecular Massively Parallel Simulator
(LAMMPS) and the embedded atom method potential of Mei et al. The aim of the
study was to understand the effects of high pressures and defects on the melting
temperature. The simulations were visualised using Visual Molecular Dynamics
(VMD). The melting temperature of a perfect copper crystal, was
found to be higher than the experimentally observed value. The melting temperature as
a function of pressure was determined and compared with experiment.
Point and line defects in the form of edge and screw dislcoations were then introduced into
crystal and the melting temperature of the crystal determined. We found that the melting
temperature decreased as the defect density is increased.
Additionally, the slope of the melting temperature curve was found to decrease
as the pressure was increased and the vacancy formation energy was found to be linearly
dependent on pressure.
12:30 PM - *CM1.6.10
Barochemistry to Multifunctional High Energy Density Solids
Choong-shik Yoo 1 Show Abstract
1 , Washington State University, Pullman, Washington, United States
Many simple diatomic molecules such as nitrogen and carbon monoxide form extended “polymeric” solids under extreme conditions that can store a large sum of chemical energy in its three-dimensional network structures made of strong covalent bonds. As such, the transformation of the singly bonded polymeric solids back to diatomic nitrogen or carbon monoxide molecules can release 5-10 times the energy of TNT without any negative environmental impact. However, the practical use of these extended solids has been very much limited, because of the formidable pressure-temperature conditions and the metastability of recovered products at ambient condition. In this paper, we will describe our recent research efforts to lower the transition pressures and enhance the stability of recovered products at ambient condition.