We performed sound velocity and density measurements on polycrystalline hexagonal close-packed (hcp) iron at simultaneous high pressure and high temperature, up to 93 GPa and 1100 K. by inelastic x-ray scattering and x-ray diffraction. Our experimental results indicate that high-temperature anharmonic corrections are negligible at least up to 1100 K and that the aggregate compressional velocity V-P scales linearly with density over the pressure and temperature range of the investigation (Birch's law). The new results are compared with literature studies and we discuss the extrapolation schemes commonly used in experimental mineral physics, with specific regard to extrapolations to the Earth's core conditions. (C) 2012 Elsevier B.V. All rights reserved.

The crystal field splitting and d bandwidth of the 3d transition metal monoxides MnO, FeO, CoO and NiO are analyzed as a function of pressure within density functional theory. In all four cases the 3d bandwidth is significantly larger than the crystal field splitting over a wide range of compressions. The bandwidth actually increases more as pressure is increased than the crystal field splitting. Therefore the role of increasing bandwidth must be considered in any explanation of a possible spin collapse that these materials may exhibit under pressure.

A comprehensive P-V-T dataset for bcc-Mo was obtained at pressures up to 31 GPa and temperatures from 300 to 1673 K using MgO and Au pressure calibrants. The thermodynamic analysis of these data was performed using high-temperature Birch-Murnaghan (HTBM) equations of state (EOS), Mie-Gruneisen-Debye (MGD) relation combined with the room-temperature Vinet EOS, and newly proposed Kunc-Einstein (KE) approach. The analysis of room-temperature compression data with the Vinet EOS yields V-0 = 31.14 +/- 0.02 angstrom(3), K-T=260 +/- 1 GPa, and K-T'=4.21 +/- 0.05. The derived thermoelastic parameters for the HTBM include (partial derivative K-T/partial derivative T)(P) = -0.019 +/- 0.001 GPa/K and thermal expansion alpha - a(0) + a(1)T with a(0) - 1.55 (+/- 0.05) x 10(-5) K-1 and a(1) - 0.68 (+/- 0.07) x 10(-8) K-2. Fitting to the MGD relation yields gamma(0)=2.03 +/- 0.02 and q=0.24 +/- 0.02 with the Debye temperature (theta(0)) fixed at 455-470 K. Two models are proposed for the KE EOS. The model 1 (Mo-1) is the best fit to our P-V-T data, whereas the second model (Mo-2) is derived by including the shock compression and other experimental measurements. Nevertheless, both models provide similar thermoelastic parameters. Parameters used on Mo-1 include two Einstein temperatures Theta(E10) = 366 K and Theta(E20) - 208 K; Gruneisen parameter at ambient condition gamma(0) - 1.64 and infinite compression gamma(infinity)=0.358 with beta = 0.323; and additional fitting parameters m = 0.195, e(0) = 0.9 x 10(-6) K-1, and g 5.6. Fixed parameters include k - 2 in Kunc EOS, m(E1) - m(E2) - 1.5 in expression for Einstein temperature, and a(0) - 0 (an intrinsic anharmonicity parameter). These parameters are the best representation of the experimental data for Mo and can be used for variety of thermodynamic calculations for Mo and Mo-containing systems including phase diagrams, chemical reactions, and electronic structure. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4794127]

We report novel molecular compound formation from silane-hydrogen mixtures with intermolecular interactions unprecedented for hydrogen-rich solids. A complex H(2) vibron spectrum with anticorrelated pressure-frequency dependencies and a striking H-D exchange below 10 GPa reveal strong and unusual attractive interactions between SiH(4) and H(2) and molecular bond destabilization at remarkably low pressure. The unique features of the observed SiH(4)(H(2))(2) compound suggest a new range of accessible pressure-driven intermolecular interactions for hydrogen-bearing simple molecular systems and a new approach to perturb the hydrogen covalent bond.

We investigate the equation of state and elastic properties of hcp iron at high pressures and high temperatures using the first-principles linear-response linear-muffin-tin-orbital method in the generalized-gradient approximation. We calculate the Helmholtz free energy as a function of volume, temperature, and volume-conserving strains, including the electronic excitation contributions from band structures and lattice vibrational contributions from quasiharmonic lattice dynamics. We perform detailed investigations on the behavior of elastic moduli and equation of state properties as functions of temperature and pressure, including the pressure-volume equation of state, bulk modulus, the thermal-expansion coefficient, the Gruneisen ratio, and the shock Hugoniot. Detailed comparison has been made with available experimental measurements and theoretical predictions.

The crystal structure of a high-pressure Fe3O4 phase was determined by in situ X-ray diffraction measurements at high pressure and temperature, using an imaging plate detector and monochromatic synchrotron X-radiation. The high-pressure phase has the Pbcm space group (CaMn2O4-type structure) with cell parameters a = 2.7992(3) Angstrom, b = 9.4097(15) Angstrom, and c = 9.4832(9) Angstrom at 23.96 GPa and 823 K. Fe3+ occupies an octahedral site and Fe2+ is in an eightfold-coordinated site described as a bicapped trigonal prism. The high-pressure CaMn2O4-type Fe3O4 phase is about 6.5% more dense than the spinel form at 24 GPa.

In contrast with the previously accepted paradigm, it is now well established that molecular hydrogen may be contained within the nano-sized cavities of clathrates. Specifically, water-based clathrate hydrates can host a significant amount of H-2 within hydrogen-bonded water cages, with interesting features such as multiple cavity occupation. Additionally, clathrate hydrate analogues have been demonstrated to hold hydrogen and novel hydrogen clathrate materials are continuing to be developed. This work discusses the structures, stabilities, occupancies, and dynamics of hydrogen clathrates and highlights recent developments towards hydrogen storage. (C) 2009 Elsevier B. V. All rights reserved.

High-pressure Brillouin and Raman scattering spectroscopy and x-ray diffraction measurements were carried out on disordered Pb(Sc1/2Nb1/2)O-3, considered to be a model system for phase transitions in relaxor ferroelectrics and related materials. The observed pressure-dependent Raman spectra are unusual, with the relaxor state distinguished by broad Raman bands. Raman spectra as a function of pressure reveal a new peak at 370 cm(-1), with two peaks near 550 cm(-1) merge above 2-3 GPa, indicating a structural phase transition in this pressure range consistent with earlier dielectric measurements. A significant softening in the longitudinal acoustic mode is observed by Brillouin scattering. Both the temperature and pressure dependencies of the linewidth reveal that the longitudinal acoustic mode softening arises from electrostrictive coupling between polar nanoregions and acoustic modes. X-ray diffraction indicates that the pressure-volume compression curve changes near 2 GPa. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3369278]

A comprehensive P-V-T dataset for bcc-tungsten was obtained for pressures up to 33.5GPa and temperatures 300-1673 K using MgO and Au pressure scales. The thermodynamic analysis of these data was performed using high-temperature (HT) and Mie-Gruneisen-Debye (MGD) relations combined with the Vinet equations of state (EOS) for room-temperature isotherm and the newly proposed Kunc-Einstein (KE) EOS. The KE EOS allowed calibration of W thermodynamic parameters to the pressures of at least 300GPa and temperatures up to 4000 K with minor uncertainties (<1% in calculated volume of W). A detailed analysis of room-temperature compression data with Vinet EOS yields V-0 = 31.71 +/- 0.02 angstrom(3), K-T = 308 +/- 1 GPa, and K-T = 0 4.20 +/- 0.05. Estimated thermoelastic parameters for HT include (partial derivative K-T/partial derivative T)(P) = -0.018 +/- 0.001 GPa/K and thermal expansion alpha = a(0) + a(1)T with a(0) = 1.35 (+/- 0.04) x 10(-5) K-1 and a(1) =0.21 (+/- 0.05) x 10(-8) K-2. Fitting to the MGD relation yielded c0 1.8160.02 and q = 0.71 +/- 0.02 with the Debye temperature (theta(0),) fixed at 370-405 K. The parameters for KE EOS include two Einstein temperatures, Theta(E1o) = 314 K and Theta(E2o) = 168 K, Gruneisen parameter at ambient condition c0 1.67 and infinite compression c1 0.66, with beta = 1.16 (which is a power-mode parameter in the Gruneisen equation), anharmonicity (m = 3.57) and electronic (g = 0.11) equivalents of the Gr_ uneisen parameter, and additional parameters for intrinsic anharmonicity, a(0) = 6.2 x 10(-5) K-1, and electronic contribution, e(0) = 4.04 x 10(-5) K-1 to the free energy. Fixed parameters include k = 2 in KE EOS and m(E1) = m(E2) = 1.5 in expression for Einstein temperature. Present analysis should represent the best fit of the experimental data for W and can be used for a variety of thermodynamic calculations for W and W-containing systems including phase diagrams, chemical reactions, and electronic structure. (C)2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4799018]