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.

Whereas several clathrate-like structures are known to exist from mixtures of H-2 + H2O under pressure, the combined high-pressure and low-temperature region of the phase diagram remains largely unexplored. Here we report a combined Raman spectroscopy and synchrotron X-ray diffraction study on the low-temperature region of the phase diagram. Below similar to 120 K, the H-2 vibron originating from the clathrate 2 (C-2) phase splits into two distinct components, yet X-ray diffraction measurements reveal no structural change between room temperature and 11 K. We suggest that the two vibrons of the C-2 phase at low temperature originate from vibrational transitions of hydrogen molecules in the ground and first excited rotational energy levels. At similar to 1 GPa we observe the clathrate 1 (C-1) phase to persist to the lowest temperature measured (80 K). Upon decompression from the C-2 phase we observed the appearance of cubic ice (I-c), which converted to a new phase before trans forming to the C-1 phase. The structure of the new phase is consistent with a water framework similar to a-quartz; the structure could also be related to the tetragonal clathrate phase reported previously for nitrogen and argon guests.

We develop an all-electron quantum Monte Carlo (QMC) method for solids that does not rely on pseudopotentials, and use it to construct a primary ultra-high-pressure calibration based on the equation of state of cubic boron nitride. We compute the static contribution to the free energy with the QMC method and obtain the phonon contribution from density functional theory, yielding a high-accuracy calibration up to 900 GPa usable directly in experiment. We compute the anharmonic Raman frequency shift with QMC simulations as a function of pressure and temperature, allowing optical pressure calibration. In contrast to present experimental approaches, small systematic errors in the theoretical EOS do not increase with pressure, and no extrapolation is needed. This all-electron method is applicable to first-row solids, providing a new reference for ab initio calculations of solids and benchmarks for pseudopotential accuracy.

The stability field of siderite has been determined up to 10 GPa. Decarbonation of siderite occurs at pressures below 6 GPa with a Clapeyron slope of about 0.0082 GPa/K. At higher pressure, we observed direct melting of siderite without decarbonation. The melting temperature is about 1550 degrees C at 10 GPa. Our experimental results, compared with previous studies on the decomposition curve of magnesite, indicate that Fe has a significant effect on the stability of magnesite-siderite solid solutions under upper mantle conditions. The reaction products are strongly dependent on the oxygen fugacity of the system. The disproportionation reaction during decomposition of siderite might be an important mechanism to explain the stability of carbon as graphite (diamond) in the Earth's mantle.

We present an extensive study of the optical, electronic, and structural properties of silane (SiH4) to 150 GPa through the use of Raman spectroscopy, optical microscopy, synchrotron infrared reflectivity, optical absorption, and synchrotron x-ray diffraction measurements. To mitigate possible contamination from previously reported metal hydride formation, we performed experiments using gold-lined sample gaskets, finding molecular silane remains in the transparent and insulating P2(1)/c structure until similar to 40 GPa. Silane shows a partial loss of crystallinity above similar to 50 GPa and appears to visibly darken. The darkening is plausibly the result of a loss of molecular character with many enthalpically competitive pathways available, including decomposition, combined with the absorptive nature of the sample. Above 100 GPa we observed crystallization into structures partially consistent with the previously reported nonmolecular I (4) over bar 2d and I4(1)/a types. In the absence of decomposition, silane remains partially transparent and nonmetallic to at least 150 GPa with a band gap constrained between 0.6 and 1.8 eV. Under pressure, silane is sensitive to irradiation from x-rays and lasers, and may easily decompose into metallic silicon. We suggest that previous reports of metallization starting from molecular SiH4 arise from decomposition, and superconductivity may originate from hydrogen-doped silicon. While silane may readily decompose, the inherent metastability provides access to a wide range of path-and sample-history-dependent states and suggests a unique range of physical properties for hydrogen-rich silicon alloys.