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]

Using a two-stage light gas gun, we obtained new shock wave Hugoniot data for an iron-sulfur alloy (Fe-11.8wt%S) over the pressure range of 94-204 GPa. A least-squares fit to the Hugoniot data yields a linear relationship between shock velocity D-S and particle velocity u, D-S (km/s) = 3.60(0.14) + 1.57(0.05) u. The measured Hugoniot data for Fe-11.8wt%S agree well with the calculated results based on the thermodynamic parameters of Fe and FeS using the additive law. By comparing the calculated densities along the adiabatic core temperature with the PREM density profile, an iron core with 10 wt.% sulfur (S) provides the best solution for the composition of the Earth's outer core. Citation: Huang, H., S. Wu, X. Hu, Q. Wang, X. Wang, and Y. Fei (2013), Shock compression of Fe-FeS mixture up to 204 GPa, Geophys. Res. Lett., 40, 687-691, doi:10.1002/grl.50180.

Optical microscopy, spectroscopic and x-ray diffraction studies at high-pressure are used to investigate intermolecular interactions in binary mixtures of germane (GeH4)+hydrogen (H-2). The measurements reveal the formation of a new molecular compound, with the approximate stoichiometry GeH4(H-2)(2), when the constituents are compressed above 7.5 GPa. Raman and infrared spectroscopic measurements show multiple H-2 vibrons substantially softened from bulk solid hydrogen. With increasing pressure, the frequencies of several Raman and infrared H-2 vibrons decrease, indicating anomalous attractive interaction for closed-shell, nonpolar molecules. Synchrotron powder x-ray diffraction measurements show that the compound has a structure based on face-centered cubic (fcc) with GeH4 molecules occupying fcc sites and H-2 molecules likely distributed between O-h and T-d sites. Above ca. 17 GPa, GeH4 molecules in the compound become unstable with respect to decomposition products (Ge+H-2), however, the compound can be preserved metastably to ca. 27 GPa for time-scales of the order of several hours. (C) 2010 American Institute of Physics. [doi:10.1063/1.3505299]

We report quantitative 3D coherent x-ray diffraction imaging of a molten Fe-rich alloy and crystalline olivine sample, synthesized at 6 GPa and 1800 degrees C, with nanoscale resolution. The 3D mass density map is determined and the 3D distribution of the Fe-rich and Fe-S phases in the olivine-Fe-S sample is observed. Our results indicate that the Fe-rich melt exhibits varied 3D shapes and sizes in the olivine matrix. This work has potential for not only improving our understanding of the complex interactions between Fe-rich core-forming melts and mantle silicate phases but also paves the way for quantitative 3D imaging of materials at nanoscale resolution under extreme pressures and temperatures.

High-pressure and variable temperature single-crystal synchrotron x-ray measurements combined with first principles based molecular-dynamics simulations were used to study diffuse scattering in the relaxor ferroelectric system PbSc1/2Nb1/2O3. Constant temperature experiments show a pressure-induced transition to the relaxor phase, in which butterfly- and rod-shaped diffuse scattering occurs around the {h00} and {hh0} Bragg spots. Simulations qualitatively reproduce the observed diffuse scattering features as well as their pressure-temperature behavior and show that they arise from polarization correlations between chemically ordered regions, which in previous simulations were shown to behave as polar nanoregions. Simulations also exhibit radial diffuse scattering [elongated toward and away from Q=(000)] that persists even in the paraelectric phase; consistent with previous neutron experiments on PbMg1/3Nb2/3O3.