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[EN] High pressure X-ray diffraction, Raman scattering, and electrical measurements, together with theoretical calculations, which include the analysis of the topological electron density and electronic localization function, evidence the presence of an isostructural phase transition around 2 GPa, a Fermi resonance around 3.5 GPa, and a pressure-induced decomposition of SnSb2Te4 into the high-pressure phases of its parent binary compounds (alpha-Sb2Te3 and SnTe) above 7 GPa. The internal polyhedral compressibility, the behavior of the Raman-active modes, the electrical behavior, and the nature of its different bonds under compression have been discussed and compared with their parent binary compounds and with related ternary materials. In this context, the Raman spectrum of SnSb2Te4 exhibits vibrational modes that are associated but forbidden in rocksalt-type SnTe; thus showing a novel way to experimentally observe the forbidden vibrational modes of some compounds. Here, some of the bonds are identified with metavalent bonding, which were already observed in their parent binary compounds. The behavior of SnSb2Te4 is framed within the extended orbital radii map of BA(2)Te(4) compounds, so our results pave the way to understand the pressure behavior and stability ranges of other "natural van der Waals" compounds with similar stoichiometry., This work has been performed under financial support from the Spanish MINECO under Project MALTA-CONSOLIDER TEAM network (RED2018-102612-T) and Project FIS2017-83295-P, from Generalitat Valenciana under Project PROMETEO/2018/123. This publication is a product of the "Programa de Valoracion y Recursos Conjuntos de I+D+i VLC/CAMPUS and has been financed by the Spanish Ministerio de Educacion, Cultura y Deporte, as part of "Programa Campus de Excelencia Internacional". Supercomputer time has been provided by the Red Espanola de Supercomputacion (RES) and the MALTA cluster. J.A.S. acknowledges a "Ramon y Cajal" fellowship (RYC-2015-17482) for financial support, and E.L.D.S. acknowledges Marie Sklodowska-Curie Grant No. 785789-COMEX from the European Union's Horizon 2020 research and innovation program. We also thank ALBA synchrotron and DIAMOND light source for funded experiments.

With ${T}_{c}\ensuremath{\sim}9.6\phantom{\rule{0.16em}{0ex}}\mathrm{K}, {\mathrm{Be}}_{22}\mathrm{Re}$ exhibits one of the highest critical temperatures among Be-rich compounds. We have carried out a series of high-pressure electrical resistivity measurements on this compound to 30 GPa. The data show that the critical temperature ${T}_{c}$ is suppressed gradually at a rate of $d{T}_{c}/dP=\ensuremath{-}0.05\phantom{\rule{0.16em}{0ex}}\mathrm{K}/\mathrm{GPa}$. Using density functional theory (DFT) calculations of the electronic and phonon density of states (DOS) and the measured critical temperature, we estimate that the rapid increase in lattice stiffening in ${\mathrm{Be}}_{22}\mathrm{Re}$ overwhelms a moderate increase in the electron-ion interaction with pressure, resulting in the decrease in ${T}_{c}$. High-pressure x-ray diffraction measurements show that the ambient pressure crystal structure of ${\mathrm{Be}}_{22}\mathrm{Re}$ persists to at least 154 GPa.

Half-Heusler thermoelectric materials are potential candidates for high thermoelectric efficiency. We report high-pressure thermoelectric and structural property measurements, density functional theory calculations on the half-Heusler material TiNiSn, and an increase of 15% in the relative dimensionless figure of merit, ZT, around 3 GPa. Thermal and electrical properties were measured utilizing a specialized sample cell assembly designed for the Paris-Edinburgh large-volume press to a maximum pressure of 3.5 GPa. High-pressure structural measurements performed up to 50 GPa in a diamond-anvil cell indicated the emergence of a new high-pressure phase around 20 GPa. A first-principles structure search performed using an ab initio random structure search approach identified the high-pressure phase as an orthorhombic type, in good agreement with the experimental results.

High pressure X-ray diffraction, Raman scattering, and electrical measurements, together with theoretical calculations, which include the analysis of the topological electron density and electronic localization function, evidence the presence of an isostructural phase transition around 2 GPa, a Fermi resonance around 3.5 GPa, and a pressure-induced decomposition of SnSb

We have performed high-pressure structural, electrical, and thermal property measurements on half-Heusler thermoelectric compound, TiCoSb. High-pressure powder diffraction measurements have been performed in an angle-dispersive geometry using synchrotron x-rays up to 115 GPa using a diamond anvil cell. The results show the cubic crystal structure (F-43m) stable and no pressure induced structural phase transitions were observed over the entire pressure range. The electrical and thermal properties were studied up to 5 GPa using a Paris-Edinburgh press equipped with a specialized sample cell assembly. The compressibility, variation of thermal conductivity, Seebeck co-efficient and thermoelectric figure of merit (ZT) for the material are discussed in detail.

Vanadium diboride (VB2) with an AlB2-type structure has been synthesized at 8 GPa and 1700 K in a D-DIA-type multianvil apparatus. The obtained bulk modulus is B0 = 262(2) GPa with fixed B′ = 4.0 for VB2 via high-pressure X-ray diffraction measurements. Meanwhile, VB2 has also been demonstrated to possess a high Vickers hardness of 27.2 ± 1.5 GPa, a high thermal stability of 1410 K in air, among the highest for transition-metal borides, and an extremely low resistivity value (41 μΩ cm) at room temperature. Results from first-principles calculations regarding the mechanical and electronic properties of VB2 are largely consistent with the experimental observations and further suggest that VB2 possesses simultaneously the properties of a hard and refractory ceramic and those of an excellent electric conductor.

The high pressure structural and transport behaviors of transition metal dichalchogenides NbSexTe2−x (x = 2, 1.5) were investigated using powder X-ray diffraction and resistivity measurements. The structural results show all these title compounds remain stable under application of pressure up to 39 GPa. The lattice compressibilites were determined and show highly anisotropic behavior. A change in superconducting transition temperature dependent on composition was observed. The resistivity measurements performed under pressure for NbSe1.5Te0.5 revealed strong enhancement in transition temperature with Tc increasing from 3.35 K at ambient, to 4.4 K at 1.8 GPa, at a rate of 0.6 K/GPa.

The thermoelectric properties of polycrystalline SnTe have been measured up to 4.5 GPa at 330 K. SnTe shows an enormous enhancement in Seebeck coefficient, greater than 200 % after 3 GPa, which correlates to a known pressure-induced structural phase transition that is observed through simultaneous in situ X-ray diffraction measurement. Electrical resistance and relative changes to the thermal conductivity were also measured, enabling the determination of relative changes in the dimensionless figure of merit (ZT), which increases dramatically after 3 GPa, reaching 350 % of the lowest pressure ZT value. The results demonstrate a fundamental relationship between structure and thermoelectric behaviours and suggest that pressure is an effective tool to control them.

High-pressure studies on thermoelectric materials allow the study of the relationship between structural, elastic, and electronic properties. The High Pressure Science and Engineering Center (HiPSEC) at UNLV performs interdisciplinary research on a wide variety of materials at high pressures. One such system, CrSi2 is an indirect band gap semiconductor that has potential applications in solar cells.

We have studied the high-pressure structural behavior of zinc ferrite (ZnFe2O4) nanoparticles by powder X-ray diffraction measurements up to 47 GPa. We found that the cubic spinel structure of ZnFe2O4 remains up to 33 GPa and a phase transition is induced beyond this pressure. The high-pressure phase is indexed to an orthorhombic CaMn2O4-type structure. Upon decompression the low- and high-pressure phases coexist. The compressibility of both structures was also investigated. We have observed that the lattice parameters of the high-pressure phase behave anisotropically upon compression. Further, we predict possible phase transition around 55 GPa. For comparison, we also studied the compression behavior of magnetite (Fe3O4) nanoparticles by X-ray diffraction up to 23 GPa. Spinel-type ZnFe2O4 and Fe3O4 nanoparticles have a bulk modulus of 172 (20) GPa and 152 (9) GPa, respectively. This indicates that in both cases the nanoparticles do not undergo a Hall-Petch strengthening. Fil: Ferrari, Sergio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long". Universidad de Buenos Aires. Facultad de Ingeniería. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long"; Argentina Fil: Kumar, R. S.. University of Nevada; Estados Unidos Fil: Grinblat, Florencia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long". Universidad de Buenos Aires. Facultad de Ingeniería. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long"; Argentina Fil: Aphesteguy, Juan Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long". Universidad de Buenos Aires. Facultad de Ingeniería. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long"; Argentina Fil: Saccone, Fabio Daniel. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Errandonea, D.. Universidad de Valencia; España

Lithium-rich anti-perovskites (LiRAPs), with general formula Li3OX (X = Cl, Br), recently reported as superionic conductors with 3-dimensional Li+ migrating channels, are emerging as promising candidates for solid electrolytes in all-solid-state lithium-ion batteries (LIBs). However, great challenges remain in the fabrication of pure LiRAPs due to difficulties such as low yield, impurity phases, thermodynamic instabilities, and moisture-sensitivity. In this work, we thoroughly studied the formation mechanism of Li3OCl and Li3OBr using various solid-state reaction routes. Different experimental strategies were developed to improve the syntheses, namely, for the purposes of phase stability, phase purity, and large-scale production. One feasible method is to use the strong reducing agents Li metal or LiH to eliminate the OH species. The results show that LiH is more effective than Li metal, mainly due to negatively charged H− and reaction uniformity. The other successful method employs a solid diffusion approach using Li2O and LiX as the starting reagents, thereby avoiding OH entirely; ball milling of reagents under Ar atmosphere was utilized to decrease initial grain size and increase the reaction rate. Fourier transform infrared spectroscopy (FTIR), thermal analyses, and first-principles calculations were performed to give indications on the reaction pathway.

We have investigated the structural behavior of CuSbS2 and CuSbSe2 thermoelectric materials under high pressure conditions up to 80 GPa using angle dispersive X-ray diffraction in a diamond anvil cell (DAC). We have also performed high pressure Raman spectroscopy measurements up to 16 GPa. We observed a pressure-induced structural transformation from the ambient orthorhombic structure with space group Pnma to a triclinic type structure with space group P1 beginning around 8 GPa in both samples and completing at 13 GPa and 10 GPa in CuSbS2 and CuSbSe2, respectively. High pressure Raman experiments complement the transitions observed by high pressure X-ray diffraction (HPXRD). The transitions were found to be reversible on releasing the pressure to ambient in the DAC. The bulk modulus and compressibility of these materials are further discussed.

We report on high-pressure X-ray diffraction measurements up to 51.2 GPa in PbCrO 4 at room temperature. Three high-pressure phases with structures different than the ambient-pressure monazite-type ( P 2 1 / n ) are reported. One phase transition was found at 3.8 GPa to an isomorphic structure to monazite. A second transition occurs at 11.1 GPa. After this transition, the coexistence of tetragonal ( I 4 1 / a ) and monoclinic ( P 2 1 ) structures is detected up to 21.1 GPa. Beyond this pressure and up to 51.2 GPa, only the high-pressure monoclinic phase is observed. Upon decompression all structural changes are reversible. Finally, the axial compressibilities for the different phases have been determined as well as the equations of state.

The crystal structure and spectral properties of bulk MoS2 were investigated at high pressures up to 51 GPa using a diamond anvil cell with synchrotron radiation in addition to high temperature X-ray diffraction and high pressure Raman spectroscopic analysis. While the crystal structure of MoS2 is stable on increasing temperature, results of high pressure experiments show a pressure-induced isostructural hexagonal distortion to a 2Ha-hexagonal P63/mmc phase around 26 GPa as predicted by theoretical calculations reported earlier. The 2Ha-hexagonal phase coexists with the ambient 2Hc phase up to 51 GPa, the highest pressure achieved in our experiments. The Raman data obtained in our high pressure experiments show consistent changes in the vibrational modes. Furthermore, the diffraction data obtained for the shocked MoS2 to pressures 8 GPa is found to be structurally resilient.

We report a high-pressure study of monoclinic monazite-type SrCrO4 up to 26 GPa. Therein we combined x-ray diffraction, Raman and optical-absorption measurements with ab initio calculations, to find a pressure-induced structural phase transition of SrCrO4 near 8-9 GPa. Evidence of a second phase transition was observed at 10-13 GPa. The crystal structures of the high-pressure phases were assigned to the tetragonal scheelite-type and monoclinic AgMnO4-type structures. Both transitions produce drastic changes in the electronic band gap and phonon spectrum of SrCrO4. We determined the pressure evolution of the band gap for the low-pressure and high-pressure phases as well as the frequencies and pressure dependences of the Raman-active modes. In all three phases most Raman modes harden under compression; however the presence of low-frequency modes which gradually soften is also detected. In monazite-type SrCrO4, the band gap blue-shifts under compression, but the transition to the scheelite phase causes an abrupt decrease of the band gap in SrCrO4. Calculations showed good agreement with experiments and were used to better understand the experimental results. From x-ray diffraction studies and calculations we determined the pressure dependence of the unit-cell parameters of the different phases and their ambient-temperature equations of state. The results are compared with the high-pressure behavior of other monazites, in particular PbCrO4. A comparison of the high-pressure behavior of the electronic properties of SrCrO4 (SrWO4) and PbCrO4 (PbWO4) will also be made. Finally, the possible occurrence of a third structural phase transition is discussed., Comment: 41 pages, 9 figures, 7 tables

Chromium silicide (CrSi 2 ) thermoelectric thin films with two different thicknesses, 1 μm and 0.1 μm, were deposited using radio frequency magnetron sputtering on glass substrates. These films were characterized after deposition and then after 300–600 °C anneals using X-ray diffraction and Energy dispersive X-ray spectroscopy. The Seebeck coefficient and electrical resistivity were measured. The compositions of the sputtered films were found to be close to the sputtering target stoichiometry. The annealing conditions and variations of thickness had a great influence on the thermoelectric performance of the films. The 0.1 μm p-type films annealed in an argon atmosphere at 400 °C exhibited the largest power factor of 1.0 × 10 − 3 W/(K 2 ·m).

In this study, the electrical resistivity and magnetization of a single crystal of Ce2Ni3Ge5 heavy fermion compound were performed under pressure. The resistivity and magnetization showed two antiferromagnetic transitions at ambient pressure. On applying pressure, the transitions merged at 1 GPa. At higher pressures, the antiferromagnetic transition temperature decreases, and disappears. It is suggesting that the critical pressure of Ce2Ni3Ge5 was 4.1 GPa.

The crystal structure of A-15 superconductor Nb 3 Ga with a critical temperature T c =16.5 K has been investigated by high pressure x-ray diffraction (HPXRD) using synchrotron x-rays and a diamond anvil cell under Ne pressure medium. The high pressure structural results indicate that Nb 3 Ga is stable up to 41 GPa. The P – V plot shows an anomaly around 15 GPa even though there are no pressure induced structural transitions are observed. High pressure resistance measurements were performed up to 0.5 GPa to understand the variation of T c under pressure. Our results show a positive pressure effect on T c .

In this paper, the effect of pressure on the transport and structural properties of GaTe, InTe, and InGaTe 2 is reported. All materials were found to exhibit pressure-induced structural transitions. For GaTe, the ambient structure transforms to a mixed state at 8 GPa, which transforms at 15 GPa to an ordered state. InTe, which crystallizes in a tetragonal structure, shows transformations at 6 and 14 GPa. InGaTe 2 , also initially tetragonal, undergoes two transitions at 9.25 GPa and 13 GPa. These transitions have shown noticeable effects on the transport properties. In particular, the Seebeck coefficient for the solid solution changes sign at the first phase transition. From these results, the thermoelectric figure of merit has been evaluated for each material. For some of these materials (InTe and InGaTe 2 ) this yields a lower efficiency than ambient conditions. However, for GaTe, this has been shown to increase the figure of merit by 14 times to 8 GPa.

A new sample cell assembly design for the Paris-Edinburgh type large-volume press for simultaneous measurements of X-ray diffraction, electrical resistance, Seebeck coefficient and relative changes in the thermal conductance at high pressures has been developed. The feasibility of performingin situmeasurements of the Seebeck coefficient and thermal measurements is demonstrated by observing well known solid–solid phase transitions of bismuth (Bi) up to 3 GPa and 450 K. A reversible polarity flip has been observed in the Seebeck coefficient across the Bi-I to Bi-II phase boundary. Also, successful Seebeck coefficient measurements have been performed for the classical high-temperature thermoelectric material PbTe under high pressure and temperature conditions. In addition, the relative change in the thermal conductivity was measured and a relative change in ZT, the dimensionless figure of merit, is described. This new capability enables pressure-induced structural changes to be directly correlated to electrical and thermal properties.

This paper presents ambient and high pressure measurements of transport properties of the Bi2Te3–Sb2Te3 series of materials. The electrical resistivity, thermal conductivity, and Seebeck coefficient have been measured on both end compounds and the direct solid solution of the two at pressure up to 10 GPa. An additional discussion involving the high pressure structure will be presented. From this, it was determined that these materials undergo at least two structural phase transitions between 0 and 20 GPa and a discussion is presented regarding this and the changes in the transport properties.

The high-pressure transport properties have been determined for nickel and PbTe. Nickel shows a reduction in electrical resistivity, an increase in thermal conductivity, and a variable effect on the Seebeck coefficient with pressure. In PbTe, a dramatic decrease in resistivity and a slow increase in thermal conductivity have been observed with increasing pressure. The three transport properties in PbTe are affected by a pressure-induced structural phase transition. The measurements show that the high-pressure phase is likely a more effective thermoelectric material than the ambient-pressure phase.

The crystal structure and the Yb valence of the YbFe2Ge2 heavy fermion compound was measured at room temperature and under high pressures using high-pressure powder X-ray diffraction and X-ray absorption spectroscopy via both partial fluorescence yield and resonant inelastic X-ray emission techniques. The measurements are complemented by first-principles density functional theoretical calculations using the self-interaction corrected local spin density approximation investigating in particular the magnetic structure and the Yb valence. While the ThCr2Si2-type tetragonal (I4/mmm) structure is stable up to 53 GPa, the X-ray emission results show an increase of the Yb valence from v = 2.72(2) at ambient pressure to v = 2.93(3) at similar to 9 GPa, where at low temperature a pressure-induced quantum critical state was reported.

MCu 2 O 3 (M=Ca and Co) system has two-leg spin ladder structure similar to that of the prototype SrCu 2 O 3 system except that the rungs are buckled with an angle of 123° and 105° for CaCu 2 O 3 and CoCu 2 O 3 compounds, respectively. We have synthesized powder samples of (Ca 1− x Co x )Cu 2 O 3 ( x =0.00–1.00) by the solid state reaction method and their structural and magnetic properties have been investigated. All the synthesized compounds crystallize in orthorhombic structure with space group Pmmn. Lattice parameters of (Ca 1− x Co x )Cu 2 O 3 decrease with the increase in Co content. DC magnetic susceptibility χ ( T ) results of the end products CaCu 2 O 3 and CoCu 2 O 3 show antiferromagnetic transition ( T N ) at 27 and 215 K, respectively. Co doping into (Ca 1− x Co x )Cu 2 O 3 enhances its T N systematically with increasing Co concentration. The χ ( T ) of CoCu 2 O 3 shows a broad transition with the peak temperature around 215 K and it was found to be field independent up to 90 kOe. The ambiguity concerning the transition was ruled out by recording the temperature dependent X-ray diffraction pattern on CoCu 2 O 3 system, which indicated that there is no structural transition in the investigated temperature range of 115–300 K. Further, specific heat measurement on CoCu 2 O 3 confirms the magnetic phase transition by the appearance of a sharp peak at 215 K.

High-pressure studies on strongly correlated-electron systems allow the study of the relationship between structural, elastic, electronic, and magnetic properties of d-and f-band systems. The High Pressure Science and Engineering Center (HiPSEC) at UNLV performs interdisciplinary research on a wide variety of materials at high pressures. One such system, YbB2 displays antiferromagnet order at ambient pressure. We present heat capacity measurements at high magnetic fields to 9 T and structural measurement at pressures up to 5 GPa on YbB2.

The crystal structure of NH3BH3 was investigated using synchrotron high pressure X-ray diffraction (HPXRD) up to 27 GPa and neutron diffraction up to 5 GPa. Density functional theoretical (DFT) calculations were carried out simultaneously for comparison. The results confirm a pressure induced phase transition from the tetragonal I 4 mm phase to a high pressure orthorhombic Cmc21 phase around 1.22 GPa. Further increase of pressure above 8 GPa, we observed a second structural transition from Cmc21 to a triclinic P1 phase which are reversible with small hysteresis. The transition pressures and the bulk modulus obtained experimentally are in good agreement with theory.

The high-figure-of-merit thermoelectric material AgPb18SbTe20 has been investigated by in situ angular-dispersive x-ray diffraction (XRD) and x-ray absorption fine-structure (XAFS) measurements up to 30 GPa. Resistivity and thermopower were measured with Bridgman-type opposed metal anvil cells. The results show that the ambient cubic \( \left( {Fm\overline{3} m} \right) \) structure transforms to orthorhombic (Pnma) at 6.4 GPa and then to the CsCl-type \( \left( {Pm\overline{3} m} \right) \) structure at 15 GPa. The ambient cubic \( \left( {Fm\overline{3} m} \right) \) phase is found to be recoverable on releasing the pressure. The thermoelectric power is found to increase with pressure for the cubic phase. The XAFS studies performed at the Pb L3-edge and Ag K-edge along with resistivity studies complement the XRD findings.

The crystal structure of Na0.75CoO2 was studied at ambient and low temperatures down to 10 K at pressures up to 40 GPa using synchrotron x-rays and a diamond cell in angle dispersion geometry. A reduction in the c / a ratio was observed at both conditions with the application of pressure. An increase in Co–O bond lengths and a decrease in Na–O bond lengths were observed above 10 GPa. The results of the density functional calculations performed agree well with the pressure induced bond length changes. The anomalous change in the c / a ratio and bond lengths indicate a pressure induced isostructural phase transition above 10 GPa. Bulk modulus calculations show this compound is less compressible than its hydrated analogues.

In situ high pressure synchrotron powder X-ray diffraction and Raman techniques were used to study the structural stability of RbBH4 at room temperature up to 27 GPa. Two successive pressure induced structural transitions were observed in RbBH4 from the ambient cubic F m 3 ¯ m phase to an orthorhombic Pnma phase around 2.5 GPa, and then to a monoclinic P2/C phase above 8 GPa. The ambient cubic phase is found to be reversible. The experiments show that the structural polymorphs of RbBH4 observed at high pressures are different from the similar cubic hydrogen storage compounds NaBH4 and KBH4.

The crystal structure of Li 3 AlH 6 was investigated at high pressures upto 27 GPa using a diamond anvil cell with synchrotron radiation in addition to high temperature X-ray diffraction. Density functional theoretical (DFT) calculations were performed simultaneously. While the structure of Li 3 AlH 6 is stable on increasing temperature, the results of high pressure experiments show a pressure induced phase transition from the ambient R 3 ¯ phase to a high pressure cubic Ia 3 ¯ phase around 10.6 GPa. The transition pressure of 10.6 GPa and the bulk modulus value B 0 = 32(2) GPa for the R 3 ¯ phase obtained are in good agreement with the theoretical results.

Angle-dispersive high-pressure powder X-ray diffraction, high-pressure Raman, and ab initio density functional theoretical (DFT) calculations were performed on KBH4 up to 20 GPa. We demonstrate that KBH4 exhibits structural phase transitions from the ambient α-KBH4 phase (cubic Fm3m) to β-KBH4 (tetragonal P421c) at 3.8 GPa and to the γ-KBH4 phase (orthorhombic Pnma) at 6.8 GPa, which is similar to the phase transition sequence observed for NaBH4 earlier. The transition pressure and bulk modulus obtained for KBH4 in our experiments are in good agreement with theoretical calculations. The results further show that the ambient pressure phase of this hydride is reversible.

We have investigated pressure-induced structural transitions in NaBH4 through density-functional theory calculations combined with X-ray and neutron diffraction experiments. Our calculations confirm that the cubic phase is stable up to 5.4 GPa and an orthorhombic phase occurs above 8.9 GPa, as observed in X-ray diffraction experiments. Both the calculations and X-ray diffraction measurements identify an intermediate tetragonal phase that appears between 6 and 8 GPa; that is, between the cubic and orthorhombic phases. This result is also confirmed by high-pressure neutron diffraction experiments performed on NaBD4. Our calculations and X-ray diffraction measurements show that the space group of the orthorhombic phase above 8.9 GPa is Pnma and the orthorhombic phase remains stable up to 30 GPa. The calculated equations of state are in excellent agreement with experiments.

Single wall carbon nanotubes (SWCNT) were ball milled with Ti and TiH 2 . Samples collected at different milling times were characterized by X-ray diffraction, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray Raman spectroscopy (XRS). While the intensity of the π ⁎ excitonic transition of the Ti+SWCNT samples remains unchanged with milling time, a doublet feature observed around 285.4 eV in the X-ray Raman spectra of the 5 h milled sample shows increasing hybridization of the Ti states with the nanotubes above the Fermi level. On the other hand, in addition to a double peak nature, the XRS spectra of TiH 2 +SWCNT samples show a decrease in the π ⁎ intensity which provides clear evidence for increasing sp 3 hybridization with milling time.

X-ray Raman scattering experiments were performed on C 60 fullerenes and multi-wall carbon nanotubes (MWCNT) up to 20 GPa and 25 GPa using a diamond anvil cell and synchrotron radiation at HPCAT, Advanced Photon Source. The intensity of the near edge peaks representing sp 2 hybridization (π⁎) decreases with increasing pressure in both materials. Around 13 GPa the X-ray Raman spectra of C 60 completely transformed to sp 3 type bonding leading to diamond-like phase. Similar features have been observed in MWCNT at 16 GPa indicating the formation of strong interlayer covalent bonds. Our experiments are in excellent agreement with recent theoretical simulations reported on the mechanical behaviour of compressed nanotubes and provide direct experimental evidence for bond switching of these carbonaceous systems at high pressures.

The structure of nanocrystalline and bulk polycrystalline ZnO were examined up to 85 GPa and 50 GPa, respectively using synchrotron X-rays and diamond anvil cells at ambient conditions. The transition from the wurtzite to the rock salt phase in the nano-ZnO takes place at 10.5 GPa; this transition pressure is 1.5 GPa higher than in bulk ZnO. A large volume collapse of about 17.5% is observed during the transition in both systems. The rocksalt phase is stable and no structural transitions are observed for both compounds at higher pressures up to the experimental limit. On decompression the rocksalt phase is found to co-exist with the wurtzite phase at ambient conditions for the nano-ZnO.

The structure of Th 2 Ti 2 O 7 pyrochlore was investigated at high pressures up to 24 GPa at cryogenic temperatures down to 6.5 K using angular dispersive X-ray diffraction with synchrotron radiation at HPCAT, Advanced Photon Source. The cell parameters were obtained by performing full profile Rietveld refinements of the diffraction data. The equation of state is obtained at low temperatures by fitting the pressure-volume data to a second order Birch Murnaghan eqation and a bulk modulus value of 168(4) GPa is obtained. The results show persistance of the pyrochlore structure up to the maximum pressure studied in the experiment and further indicate that pressure induces solely magnetic ordering which do not involve crystal symmetry changes.

Recent experimental measurements and ab initio calculations in scheelite CaWO 4 , SrWO 4 , BaWO 4 , PbWO 4 , EuWO 4 and YLiF4 crystals reveal the existence of complex high-pressure phase diagrams, which present striking differences but also relevant similarities. In this work we show that the high-pressure structural sequence in the studied scheelites can be understood on the basis of the positions of the different ABX4 compounds in Fukunaga and Yamaoka's diagram and in Bastide's diagram. Our study can help to understand the phase diagrams and high-pressure phase transitions occurring in ABX4 compounds with scheelite, wolframite, fergusonite, zircon, or pseudoscheelite structures.

The structural properties of CaWO{sub 4}, SrWO{sub 4}, BaWO{sub 4}, PbWO{sub 4}, and EuWO{sub 4} scintillating crystals under pressure have been studied by X-ray powder diffraction, X-ray absorption near-edge structure measurements, Raman spectroscopy, and ab initio density functional theory calculations. The results obtained from these studies will be reviewed here and their differences and similitudes discussed.

We have performed in situ high pressure X-ray powder diffraction experiments on nanocrystalline BaTiO 3 (average particle size 40 nm) up to 52 GPa using synchrotron X-rays in the angle dispersive geometry with a diamond anvil cell. In order to understand the grain size effect on the tetragonal (P4mm) to cubic (Pm3m) phase transition and the compressibility, experiments were also carried out on bulkBaTiO 3 up to 36 GPa. The results show nanoBaTiO 3 is less compressible than the bulkBaTiO 3 and the phase transition pressure is found to be 1.9 GPa higher for nanoBaTiO 3 .

In this paper, we combine a theoretical study of the structural phases of CaWO 4 and SrWO 4 under high pressure along with the results of angle-dispersive X-ray diffraction (ADXRD) and X-ray absorption near-edge structure (XANES) measurements of both tungstates up to approximately 20 GPa. The theoretical study was performed within the ab initio framework of the density functional theory (DFT) using a plane-wave basis set and the pseudopotential scheme, with the generalized gradient approximation (GGA) for the exchange and correlation contribution to the energy. Under normal conditions, CaWO 4 and SrWO 4 crystallize in the scheelite structure. Our results show that in a hydrostatic environment, both compounds undergo a scheelite-to-fergusonite phase transition with increasing pressure. We present a comparison of the evolution of the structural parameters, equation of state, and of the features of the transition, finding an overall excellent agreement between the experimental and theoretical results.

The structure of bilayer manganite LaSr2Mn2O7 has been investigated under high pressure using synchrotron and laboratory X-rays in the angle-dispersive X-ray diffraction geometry up to 35 GPa at ambient temperature. The Rietveld refinement results of the powder diffraction data show a crossover between the axial and equatorial Mn–O bond length values and a significant suppression of Jahn–Teller (JT) distortion around 16 GPa. Our results imply that a re-entrant behavior in LaSr2Mn2O7, which shows equality in all the three Mn–O bond length values around 16 GPa similar to ambient conditions, may dramatically alter the orbital ordering. Further, the P-V data for the bilayer compounds indicate that the additional substitution of La3+ for Sr2+ lowers the bulk modulus and enhances lattice softening.

The properties of a silicone fluid with initial viscosity of 1 cst as a pressure transmitting medium for diamond anvil cells have been determined by ruby R1 line broadening and R1-R2 separation measurements to 64 GPa at ambient temperature. By these criteria, the silicone fluid is as good a pressure medium as a 4:1 methanol:ethanol mixture at low pressures to about 20 GPa, and is better than the mixture at higher pressures. Although argon media are better than the silicone at pressures to 30 GPa, this silicone behaves as well as argon at higher pressures. Furthermore, the silicone is easier to load than argon and is almost chemically inert.

We have studied the effect of pressure on the superconducting transition temperature (Tc) of thorium doped La1–xThx FeAsO (x = 0.2) superconductor under hydrostatic pressures up to 1.6 GPa by resistivity and magnetization experiments. Application of pressure increases the Tc to 31 K with a positive pressure coefficient of ∼1 K/GPa. Low temperature X-ray diffraction studies performed at 7.8 K at high pressures show no pressure induced structural changes and the tetragonal P4/nmm structure is found to persist up to 31 GPa. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Boron K -edge inelastic X-ray scattering experiments were performed on clean B 4 C and shock impact recovered boron carbide up to 30 GPa and at ambient temperature to understand the pressure induced bonding changes. The spectral features corresponding to the boron site in the interlinking chain remained unchanged up to 30 GPa. The results of our experiments indicate that pressure induces less distortion to the boron sites and the local amorphization observed in the previous reports are due to the rearrangement of carbon atoms under extreme conditions without affecting the boron environment.

The ternary chalcopyrite compounds CuAlSe 2 and CuAlS 2 were studied up to 14.8 GPa in a diamond anvil cell by energy dispersive X-ray diffraction (EDXRD). The powder X-ray diffraction data at high pressure show that CuAlSe 2 undergoes a pressure-induced structural phase transition around 12.5 GPa from the chalcopyrite structure to NaCl type structure. CuAlS 2 exhibits structural stability up to 14.8 GPa. The pressure-induced structural phase transition observed in the EDXRD experiments is consistent with the theoretical total energy calculations. The results are consistent with the conclusions of a recent high-pressure Raman study reported for these compounds.

We have performed room temperature high pressure structural studies on the ferromagnetic perovskite SrRuO 3 to 34 GPa. We have also used three different pressure media (silicone fluid, 4:1 methanol:ethanol and argon) to test for possible effects of pressure media on compression data. The orthorhombic perovskite structure is stable to the highest pressure, and the data can be fit with a bulk modulus of B 0 = 192 ( 3 ) GPa with a pressure derivative of B 0 ′ = 5.0 ( 3 ) for all of the pressure media. We have also examined the high pressure behavior of the RuO 6 octahedra using a model that assumes the octahedra are not distorted. Various tilt angles around the ideal cubic perovskite axes are found and can be used to estimate the Ru–O–Ru bond angle that is known to be directly related to the ferromagnetic Curie temperature. For all pressure media, there appears to be a minimum in the Ru–O–Ru bond angle around 15 GPa. Implications for the observed high pressure magnetic behavior of SrRuO 3 will be discussed.

We have performed resistivity measurements as a function of applied pressure to 2.0 GPa on a single crystal of UCu 2 Si 2 . We find that the ambient pressure magnetic ordering temperature of 100 K, and the low-temperature ordering at 50 K, both decrease gradually in a manner consistent with itinerant magnetism. The results will be compared to previous measurements on UCu 2 Ge 2 .

High-pressure X-ray (synchrotron) powder diffraction experiments have shown that the quasi-two-dimensional heavy fermion antiferromagnet CeRhIn 5 retains its tetragonal HoCoGa 5 -type structure up to 13 GPa both at room ( T =295 K) and low ( T =10 K) temperature. Refined structural parameters reveal unusual behavior at pressures around 4–5 GPa.

One of the best-known spin liquids is the geometrically frustrated pyrochlore Tb2Ti2O7. In a spin liquid, there are short-range dynamic magnetic correlations and an absence of static long-range magnetic order down to 0 K. We have measured heat capacity at low temperatures in magnetic field to 80 kOe on Nd2Ti2O7, Tb2Ti2O7, and Tb2TiO5 to better understand the spin liquid state. Nd2Ti2O7 displays antiferromagnetic order at T = 0.6 K much lower than the paramagnetic Curie temperature. Application of magnetic field or distortion of the Tb sublattice destroys the spin liquid as evidenced by static spins.