It was theorized, that at high compressions, the increased zero-point oscillations in hydrogen would destabilize the lattice and form a ground fluid state at 0 K1. Theory also predicts this fluid would have very unusual properties governed by quantum effects2,3. By combining Raman spectroscopy and in-situ high temperature techniques at high pressures, we have investigated the behavior of dense hydrogen above 200 GPa and above 300K, P-T conditions previously inaccessible in experiments. The novel data lead to a significant revision of the phase diagram of hydrogen above 200 GPa and suggest unusual physics in the molecular liquid and solid phases.
1.E. Brovman, Y. Kagan, A.Kholas, Soviet Physics JETP. 35, 783 (1972).
2.E. Babaev, A. Sudbo, N.W. Ashcroft, Nature. 431, 666 (2004).
3.E. Babaev, A. Sudbo, N.W. Ashcroft, Phys. Rev. Lett. 95, 105301 (2005).

Layered superconducting compounds have been the focus of immense interest due to the discovery of superconductivity in cuprate and iron-based systems. Discovery of Fe based pnictide superconductors viz. REFeAsO/F (RE=rare earths) with superconductivity in more than 55K has attracted a lot of attention. Primarily, because this is the only class of superconductors besides infamous high T_c cuprates (HTSc), which falls outside the popular BCS (Bardeen Cooper Schrieffer) theoretical limit of 40K. Moreover both Fe pnictides and Cuprates superconductors do have striking similarities viz. the semimetallic/insulating magnetic ground states, very high upper critical fields and the layered structures. Seemingly the role of CuO₂ planes in cuprates is played by the FeAs superconducting layers in pnictides. Within the same structure of REFeAsO, when FeAs layer is replaced by BiS₂ a new superconductor REOBiS₂ emerged very recently, though with lower T_c of around 5K. The similar layered structure and existence of superconductivity in BiS₂ layer mimicking the FeAs superconducting block of REFeAsO system has been a recent point of debate and it will be addressed in the presentation and comment on the fact about how the new FeAs and BiS₂ based superconductors are thought provoking for both experimentalists and theoreticians outside the HTSc cuprates mystery of more than two decades by now.
Discovery of superconductivity in BiS₂ layers of Bi₄O₄S₃ and ReO/FBiS₂ systems has attracted tremendous interest of both experimentalists and theoreticians from condensed matter physics community. Comparing the role of various Re in ReO/FFeAs and ReO/FBiS₂, one finds that though T_c of (La/Pr/Nd)O_0.5F_0.5BiS₂ is around 2.1K/3.5K/5K respectively, the same for (La/Pr/Nd)O_0.8F_0.2FeAs is 26K/43K/50K. It appears that chemical pressure plays an important role in superconductivity of these layered BiS₂ and FeAs superconductors. We review our efforts on synthesis and physical properties of title compounds from very beginning and present the new results related to impact of hydrostatic pressure on their superconductivity. The T_c of ReO/FBiS₂ compounds is enhanced to fivefold for just above 1GPa external pressure, accompanied with normal state semi-metallic to metallic transition in normal state, which is thought provoking for solid state physics community.

Uranyl peroxides are a diverse family of compounds that form in environmentally relevant conditions. They are of great importance due to the prevalence of the uranyl (UO₂²⁺) cation in nuclear waste. This oxidized form of uranium is a common constituent of nuclear waste, and peroxide forms in waste from the alpha-radiolysis of water. Understanding the properties of uranyl peroxides is considered an important piece of the nuclear waste management pie. Uranyl peroxides assemble into diverse nano-cages. These clusters consist of 20 to -68 uranyl units in various cage topologies. Their properties have been characterized extensively in ambient P-T conditions; uranyl peroxides form spontaneously in wide pH ranges, and can form a multitude of structures due to the uranyl cation's ability to incorporate a variety of ligands in its equatorial bonding positions. Much study has been devoted to characterizing their crystal structures and properties in solution. These structures often crystallize readily and persist for extended time periods in situ. Here, several uranyl peroxides have been studied in the solid-state at elevated pressures to determine their strengths, bulk moduli, and structures at high pressures. The aforementioned properties have been found to depend heavily on the compositions of the uranyl peroxides in question, and their ambient structures. Two uranyl peroxides in particular have been fully characterized: U₆₀ and U₂₄Py₁₂. U₆₀ clusters are of special interest because they are topologically identical to carbon fullerenes, forming isometric crystals with the formula: Li₆₈K₁₂(OH)₂₀[UO₂(O₂)(OH)]₆₀(H₂O)₃₁₀. In diamond anvil cell experiments, the fullerene U₆₀ topology is stable up to pressures of 10 GPa, above which the structure collapses irreversibly. A pressure-induced tetragonal structure (a = 36.6764(2), c = 35.1672(4) Å) forms in the range of 4 to 10 GPa, as determined by in situ synchrotron XRD and Raman spectroscopy. Above 10 GPa, the U₆₀ decomposes into smaller uranyl peroxides, as confirmed by ESI-MS. U₂₄Py₁₂ clusters form tetragonal crystals at ambient pressures with the formula: Na₈[(UO₂)₂₄(O₂)₂₄(P₂O₇)₁₂]. Single crystals of U₂₄Py₁₂ undergo reversible pressure-induced amorphization at 17 GPa. Based on Raman spectroscopy, the uranyl units persist to 50 GPa, at which point the Raman signal is imperceptible. Following pressure quenching, the original tetragonal structure of U₂₄Py₁₂ recovers, as evidenced by in situ XRD and Raman spectroscopy. In contrast to the topologically-identical C₆₀, which collapses at pressures above 30 GPa, but retains its hexagonal structure, U₆₀ loses its long-range periodicity at much lower pressures and ultimately forms smaller clusters. U₂₄Py₁₂ retains its chemical and structural integrity upon pressure quenching. The persistence of uranyl peroxide nano-cage structures during pressure experiments is a unique result, showing the strength of these compounds in extreme conditions.

The combination of transition metal and light elements such as B, C, N, and O is an effective approach to developing novel superhard multifunction materials. With this philosophy, compounds such as OsB₂, ReB₂, FeB₄, PrN₂, IrN₂, OsN₂, Re₃N, and Re₂N were successfully designed for the past decade. TM nitrides play a special role within these materials, for their high melting point, high chemical stability, corrosion resistance, high hardness, semiconductor property, and superconductivity with relatively high transition temperature. Thus we systematically studied several kinds TM nitrides theoretically and experimentally.
The structure, stoichiometry, phase transition, mechanical properties, hardness, and electronic structure of typical transition metal - nitrogen systems, such as Re-N, Tc-N, W-N, Nb-N, and FeN are investigated extensively by means of first-principles density functional theory. And some of them were synthesized by high pressure and high temperature technique and the high pressure characters of them were also studied. More than a dozen of new nitrides such as Re₃N₂, ReN₃, ReN₄, TcN, Tc₂N₃, TcN₂, TcN₃, TcN₄, W₃N₂, NbO-WN, W₅N₆, Nb₂N, NbN₂, NbN₃, and NbN₄ with high hardness have been theoretically designed. The polycrystalline ε-NbN sample has been synthesized, and the incompressibility and rigidity have been measured for the first time. Our works on typical transition nitrides established the relations among their physical properties such as the N concentration, bonding, and hardness. New approaches for designing novel superhard multifunctional materials are therefore covered throughout the research.

As experimentally established, all the alkali metals and heavy alkaline earth metals (Ca, Sr and Ba) become progressively less conductive on compression, at least up to some critical limit over a broad pressure range. Of these metals, Li and Na clearly undergo pressure-induced metal-insulator transitions, which may also be called reverse Mott transitions. Here, using group theory arguments and first-principles calculations, we show that such transitions are controlled by symmetry principles not previously recognized. The valence bands in the insulating states are described by simple and composite band representations constructed from localized Wannier functions centered on points unoccupied by atoms, and which are not necessarily all symmetrical. The character of the Wannier functions is closely related to the degree of s-p(-d) hybridization and reflects multi-center chemical bonding in these insulating states. The conditions under which an insulating state is allowed for structures having an integer number of atoms per primitive unit cell as well as re-entrant (i.e., metal-insulator-metal) transition sequences are detailed, resulting in predictions of semimetallic phases with flat surface states. The general principles developed are tested and applied to the alkali and alkaline earth metals, including elements where high-pressure insulating phases have been identified or reported (e.g., Li, Na, and Ca).

Materials are being designed and engineered for ever superior mechanical and operational properties, such as steels for lighter cars and energy-absorbing behavior in an accident, and titanium aluminides for lighter airplane turbine blades. The manufacturing of such materials may involve processes at extreme conditions, under high pressure or high temperature. Examples are high-pressure torsion and near net-shape forging. Therefore, it becomes eminently important to know and understand the phase diagrams of such materials at extreme conditions. Structural changes may open processing windows, while elevated mechanical properties are conserved under less extreme conditions. Here, we present first phase diagram studies on high-manganese steels and on titanium aluminides by in-situ synchrotron X-ray diffraction in a large-volume cell.

Metallic glass (MG) is an important new category of materials with many unique properties, but very few rigorous laws are known to define its ‘disordered&’ structure. Recently we found that under simple compression, the volume (V) of a MG changes precisely with the 2.5 power of its principle diffraction peak position (1/q₁). In the present study, we determine the V and q₁ of a Ce₆₈Al₁₀Cu₂₀Co₂ MG by in-situ high-pressure x-ray microscopy and diffraction, and find that the 2.5 power law holds even through and during its polyamorphic transition. The transition is, in effect, equivalent of a continuous compositional change of 4f-electron-localized Ce (γ-like) to 4f-electron-itinerant Ce (α-like), indicating the 2.5 power law is general for tuning with composition and pressure. We further reexamine the previously reported compositional power law exponent of 2.3 on seven different MGs, and find indeed the more precise determination also yields the exponent of 2.5. The exactness and universality of the 2.5 power law imply that the structural change in various MGs may strictly follow a general rule of packing which is imposed by efficient filling of the space without identifiable symmetry.

Pressure categorically and profoundly alters all materials. With the recent experimental breakthroughs after the turn of century, especially the new synchrotron facilities and nanotechnology coupled with diamond-anvil cell, sweeping changes of structural, electronic, magnetic and bonding properties have been uncovered and often come as a total surprise. Fundamental breakthroughs have been observed across the board in simple elements, organic compounds, stoichiometry of compounds, free electron metals, strongly correlated systems, superhard materials, materials, metallic glasses, nanomaterials, and deep-Earth minerals. We are witnessing a new era of materials science and applications with the added pressure dimension.

The thermal behaviour of materials exposed to high pressure process during manufacturing or transformation cannot be understood and/or predicted if their physicochemical properties are determined under atmospheric pressure conditions. The high pressure calorimetry is used to simulate the actual condition thus providing critical data on the materials&’ behaviour.
Based on the MicroDSC technology, Setaram Instrumentation has designed a calorimeter operating up to 1 000 bars and between -45°C and 120°C. A dedicated high pressure panel is attached to the HP MicroDSC to cover the pressure range either under controlled pressure scanning or isobaric conditions. A review of recent work perfroemd with this technique will be presented to illustrate the effectiveness of directly acquiring calorimetric data at high pressure.
For example polymers are often processed at elevated temperatures and pressures (ex: high pressure injection molding, extrusion) and under different gases (foaming) with significant effects on their melting points or glass transition temperatures. High pressure calorimetry can be also applied in the food industry for the investigation of the oxidative stability of oils; the modification of glass transition temperature of carbohydrates; to polymorphic changes in fatty compounds, etc.. Pressure-induced solid-solid transitions are also of key interest in the pharmaceutical industry. Gas hydrates or clathrate formation and dissociation, and wax appearance temperature in crude or diesel oils represent another pressure dependant phenomena which has been successfully studies using high pressure calorimetry as well

Metal organic frameworks (MOFs) are microporous materials consisting of metal ions and organic linkers which have a huge variety of applications like gas storage, separation, catalysis, drug delivery etc. Zeolitic imidazolate frameworks (ZIF) are a class of MOFs where the metal atoms (Mⁿ⁺=Zn²⁺, Co²⁺) are tetrahedrally coordinated to the N atoms of imidazolate derived linkers (IM=C₃N₂H₃^-) subtending an angle of 1450 at M-IM-M centre analogous to Si-O-Si angle in zeolites to form porous architecture. ZIF-8, a prototypical ZIF has been widely studied due to its exceptional chemical, thermal stability and its gas adsorption properties which makes it a potential candidate for many practical applications. ZIF-8 posses exceptionally low shear modulus (< 1 GPa) which results in pressure induced mechanical failure upon application of shear stresses when used in practical applications. We have used Brillouin spectroscopy to study the pressure dependence of the acoustic modes (or elastic properties) of ZIF-8 in different pressure transmitting mediums (guests) and under non-hydrostatic conditions. Our study shows the pressure induced flexibility and dynamics of ZIF-8 as well as a huge increase in the acoustic velocities on applying pressure, illustrating the role of guest in enhancing the elastic properties. On applying pressure the size of windows connecting the pores of ZIF-8 increases (gate opens) letting more guests to enter the pores. The pressure transmitting medium plays an important role in this gate opening behaviour. The interaction between the framework and guest which results in different pressure dependent behaviour in different mediums is investigated using high pressure Raman spectroscopy. We have also studied the temperature dependent elastic properties of ZIF-8 on adsorption of various gases.

Metal-organic frameworks (MOFs) of the general formula [cat][M(HCOO₃)], with M = divalent cation and cat = amine cation, have received a lot of attention in recent years because of their CO₂ sorption capacity, magnetic, ferroelectric and multiferroic properties, and glassy behavior.[1-4] In this respect, the first report on the discovery of multiferroic properties in dimethylammonium (DMA) MOFs of the formula [(CH3)₂NH₂][M(HCOO)₃] (M = Mn, Ni, Fe, Co) was published in 2009.5 This discovery promoted broad interest in the properties and phase transition mechanism in these compounds, as well as led to many efforts to synthesize novel amine-templated metal formate frameworks. Raman scattering and IR spectra have been performed on azetidinium zinc formate, [(CH2)3NH2][Zn(HCOO)3] under the Temperature- and pressure variation. Raman spectra reveal distinct anomalies in mode frequencies and bandwidths near 250 and 300 K. These anomalies were attributed to structural phase transitions associated with the gradual freezing of ring-puckering motions of the azetidinium cation. Pressure-dependent studies revealed a pressure-induced transition near 0.4 GPa. Raman spectra indicate that the structure of the room-temperature intermediate phase observed near 0.4 GPa is the same as the monoclinic structure observed at ambient pressure below 250 K. The second phase transition was found near 2.4 GPa. This transition has strong first-order character and is associated with strong distortion of both the zinc formate framework and azetidinium cations. The last phase transition was found near 7.0 GPa. This transition leads to lowering of the symmetry and further distortion of the zinc formate framework, whereas the azetidinium cation structure is weakly affected.
References
(1) Rossin, A.; Chierotti, M. R.; Giambiastiani, G.; Gobetto, R.; Peruzzini, M. CrystEngComm 2012, 14, 4454minus;4460.
(2) Zhang, W.; Xiong, R. G. Chem. Rev. 2012, 112, 1163minus;1195.
(3) Wang, Z.; Zhang, B.; Otsuka, T.; Inoue, K.; Kobayashi, H.; Kurmoo, M. Dalton Trans. 2004, 2209minus;2216.
(4) Jain, P.; Dalal, N. S.; Toby, B. H.; Kroto, H. W.; Cheetham, A. K. J. Am. Chem. Soc. 2008, 130, 10450minus;10451.

Molybdates and tungstates systems with A₂(XO₄)₃ formula (A = a trivalent ion from Al to Ho; X = Mo⁶⁺, W⁶⁺) have been investigated due to their optical, ferroelastic, ferroelectric and negative thermal expansion (NTE) properties.^1,2,3 Particularly, NTE systems are widely studied as potential fillers in order to control thermal expansion in composites. The Sc₂(WO₄)₃/ZrO₂ and Sc₂(WO₄)₃/Cu composites, for example, show almost zero thermal expansion when the volume fraction of Sc₂(WO₄)₃ is as high as 25 and 70%, respectively.⁴ In general, the easy of XO4 rotation in open framework systems is at the origin of its specific properties. In particular, the relation of these rotations with the low/negative thermal expansion of such structure is well established (see e.g. the NASICON and LAS frameworks, see e.g. J. Mol. Struct. 270 (1992) 407-416, Solid State Ionics 9&10 (1983) 845-850, 73 (1994) 209-220, etc. and refs herein) , i.e., NTE properties in framework structures are intrinsically related to the existence of low energy Rigid Unit Modes (RUMs) or transverse anharmonic vibrations of the two-fold coordinated oxygen atoms. In this work we report results of high-pressure Raman experiments (P < 8 GPa) on In_2-xY_xMo₃O₁₂ for x = 0.0 and 0.5. A crystalline to crystalline structural phase transition and pressure-induced amorphization (PIA) have been identified. The structural phase transition takes place at 1.5 and 1.0 GPa for In₂(MoO₄)₃ and In_1.5Y_0.5(MoO₄)₃, respectively, resulting in the change of structure from monoclinic P2₁/a to a more denser structure. The pressure-induced amorphization started at 5 GPa and 3.4 GPa for In₂Mo₃O₁₂ and In_1.5Y_0.5Mo₃O₁₂, respectively. The amorphization process takes place in two stages in the case of In_1.5Y_0.5Mo₃O₁₂ phase, while for In₂Mo₃O₁₂ it is not complete until the pressures as high as 7 GPa. Our results also suggest that with increase of ionic size of the A³⁺ ions, the octahedral distortion increases and consequently larger local structural disorder is introduced in the A₂(MoO₄)₃ system.
References
[1] E. T. Keve, S. C. Abrahams and J. L. Bernstei, Journal of Chemical Physics. 1971, 54, 3185-&.
[2] Y. Wang, T. Honma and T. Komatsu, Materials Chemistry and Physics. 2012, 133, 118.
[3] J. S. O. Evans, T. A. Mary and A. W. Sleight, Journal of Solid State Chemistry. 1997, 133, 580.
[4] Q. Q. Liu, X. N. Cheng and J. Yang, Materials Technology. 2012, 27, 388.

The discovery in ZrW₂O₈ [1] of strong and isotropic negative thermal expansion (NTE), also known as thermomiotic behavior [2], has raised interest in searching for materials with similar NTE properties. The NTE phenomenon has been the subject of manystudies with the aim to find suitable materials for applications that require controllable, low, or zero thermal expansion coefficients [3].
Among the NTE materials, the family of A₂M3 O₁₂ (A = trivalent cation, M = W/Mo) compounds has been widely investigated, since many members exhibit such a phenomenon [3]. The NTE behavior in Y₂Mo₃O₁₂ is known but the mechanism is not yet satisfactorily understood [4]. The importance of understanding the pressure-induced amorphization in these materials comes from the fact that this phenomenon is related to NTE [5].
In this work we investigate the vibrational properties a high-pressure Raman spectroscopic study of Y₂Mo₃O₁₂ and Y₂Mo₃O_12middot;xH₂O (x < 3). Our goal is to achieveinsight concerning the role of MoO₄ units and O-O bonds on the structural packing and related NTE phenomenon. An analysis of vibrational modes suggests that the anhydrous material experiences a phase transformation from orthorhombic to a lower symmetry phase (probably monoclinic) about 0.3 GPa, and to a highly disordered phase above 2.4 GPa. The structural transformation to the high-pressure disordered phase is not reversible and suggests the onset of a pressure-induced amorphization process. The vibrational mode dependence on pressure is discussed considering lattice dynamics calculations.
Reference
[1] J. Evans, Z. Hu, J. Jorgensen, D. Argyriou, S. Short, A. Sleight, Science 275 (1997) 61.
[2] C.P. Romao, K.J. Miller, C.A. Whitman, M.A. White, B.A. Marinkovic, in: Jan Reed-ijk, K. Poeppelmeier (Eds.) Comprehen. Inorg. Chem. 4 (2013) 127.
[3] J. Evans, T. Mary, A. Sleight, Physica B 241 (1997) 311.
[4] B.A. Marinkovic, P.M. Jardim, R.R. de Avillez, F. Rizzo, Solid State Sci. 7 (2005)1377.
[5] S. Sikka, J. Phys. Condens. Matter 16 (2004) S1033.

Germanates have wide applications as structural analogs in the study of silicate crystals and glasses, particularly in the field of high-pressure research. At high pressures and temperatures, (Mg,Fe)(Si,Ge)O₃ undergoes a sequence of transitions from a pyroxene (px) to a perovskite (pv), followed ultimately by a transition to a CaIrO₃-type post-perovskite (ppv) structure. In silicates, this sequence of transitions is crucial for understanding the structure and dynamics of planetary interiors, but the transition pressures and temperatures are difficult to achieve (e.g. ~125 GPa and 2500 K pv agrave; ppv in MgSiO₃). Magnesium iron germanates are useful analogs because they undergo these phase transitions at much lower pressures and temperatures than in the silicates (e.g. ~63 GPa and 1800 K pv agrave; ppv in MgGeO₃), enabling improved characterization of the nature and properties of the transitions. In order to investigate the effect of Fe²⁺ on the pv agrave; ppv phase boundary, and on the compressibilities of pv and ppv, polycrystalline px with compositions of (Mg_xFe_1-x)₂Ge₂O₆ (x = 1, 0.9, 0.8, 0.6, 0.5) were synthesized and characterized using x-ray diffraction, Raman, Mössbauer, and microprobe analysis. The px samples were found to exhibit a linear increase in lattice parameters with iron content. Raman peaks became broader, and shifted linearly to lower frequency with increasing iron. Mössbauer data for (Mg_0.5Fe_0.5)GeO₃ and (Mg_0.8Fe_0.2)GeO₃ px showed that the iron is high-spin Fe²⁺ that is concentrated primarily in the M1 site. Laser heated x-ray diffraction diamond anvil cell compression experiments were performed at beamline 13-ID-D of the Advanced Photon Source. Pv and ppv were synthesized from all px compositions upon compression and heating. The pressure-volume equations of state of MgGeO₃ and (Mg_0.8Fe_0.2)GeO₃ pv and ppv were determined, showing a change in volume between the compositions but negligible effect of Fe²⁺ on compressibility. Future work will focus equation of state measurements of other compositions in order to further determine the effect of Fe on the compressibility of the pv and ppv phases, on the exact temperature and pressure range for the pv agrave; ppv phase transition in each compositi