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.

A planetary interior is under high-pressure and high-temperature conditions and it has a layered structure. There are two important processes that led to that layered structure, (1) percolation of liquid metal in a solid silicate matrix by planet differentiation, and (2) inner core crystallization by subsequent planet cooling. We conduct high-pressure and high-temperature experiments to simulate both processes in the laboratory. Formation of percolative planetary core depends on the efficiency of melt percolation, which is controlled by the dihedral (wetting) angle. The percolation simulation includes heating the sample at high pressure to a target temperature at which iron-sulfur alloy is molten while the silicate remains solid, and then determining the true dihedral angle to evaluate the style of liquid migration in a crystalline matrix by 3D visualization. The 3D volume rendering is achieved by slicing the recovered sample with a focused ion beam (FIB) and taking SEM image of each slice with a FIB/SEM crossbeam instrument. The second set of experiments is designed to understand the inner core crystallization and element distribution between the liquid outer core and solid inner core by determining the melting temperature and element partitioning at high pressure. The melting experiments are conducted in the multi-anvil apparatus up to 27 GPa and extended to higher pressure in the diamond-anvil cell with laser-heating. We have developed techniques to recover small heated samples by precision FIB milling and obtain high-resolution images of the laser-heated spot that show melting texture at high pressure. By analyzing the chemical compositions of the coexisting liquid and solid phases, we precisely determine the liquidus curve, providing necessary data to understand the inner core crystallization process.

Binary mixtures of hydrogen and ammonia were compressed in diamond anvil cells to 15 GPa at room temperature over a range of compositions. The phase behavior was characterized using optical microscopy, Raman spectroscopy, and synchrotron X-ray diffraction. Below 1.2 GPa we observed two-phase coexistence between liquid ammonia and fluid hydrogen phases with limited solubility of hydrogen within the ammonia-rich phase. Complete immiscibility was observed subsequent to the freezing of ammonia phase III at 1.2 GPa, although hydrogen may become metastably trapped within the disordered face-centered-cubic lattice upon rapid solidification. For all compositions studied, the phase III to phase IV transition of ammonia occurred at similar to 3.8 GPa and hydrogen solidified at similar to 5.5 GPa, transition pressures equivalent to those observed for the pure components. A P-x phase diagram for the NH(3)-H(2) system is proposed on the basis of these observations with implications for planetary ices, molecular compound formation, and possible hydrogen storage materials.

Nanocasting at high pressure has been recently proposed as a novel strategy for the synthesis of periodic mesoporous materials with crystalline walls. In this study we present results on the synthesis of mesostructured stishovite from mesostructured FDU-12/carbon composite precursor using the multi-anvil press. Results from quenched experiments performed at a pressure of 14 GPa indicate that a minimum temperature of 500 degrees C is needed to crystallize stishovite from the amorphous silica precursor with a preserved mesostructure. Transmission electron microscopy combined with small angle X-ray scattering measurements confirmed the mesostructure of synthetic stishovite having carbon-filled pores with a diameter of similar to 19 nm similar to the pore size of the FDU-12 precursor. Calcination of the stishovite/carbon composite at 450 degrees C in air at ambient condition leads to amorphization of the stishovite. Our results show that mesostructure materials can be synthesized at very high pressures without loss or critical modification of the mesostructure. (C) 2014 Elsevier Inc. All rights reserved.

Our first-principles calculations show that both the compressional and shear waves of epsilon-Fe become elastically isotropic under the Earth's inner core conditions, with the variation in sound velocities along different angles from the c axis within 1%. We computed the thermoelasticity at high pressures and temperatures from quasiharmonic linear response linear-muffin-tin-orbital calculations in the generalized-gradient approximation. The calculated anisotropic shape and magnitude at ambient temperature agree well with previous first-principles predictions, and the anisotropic effects show strong temperature dependences. This implies that other mechanisms, rather than the preferential alignment of the epsilon-Fe crystal along the Earth's rotation axis, account for the seismic P-wave travel time anomalies. Either the inner core is not epsilon-Fe, and/or the seismologically observed anisotropy is caused by inhomogeneity, i.e., multiple phases. Citation: Sha, X., and R. E. Cohen (2010), Elastic isotropy of epsilon-Fe under Earth's core conditions, Geophys. Res. Lett., 37, L10302, doi:10.1029/2009GL042224.

Hydrogen sulfide (H2S) and hydrogen (H-2) crystallize into a 'guest-host' structure at 3.5 GPa and, at the initial formation pressure, the rotationally disordered component molecules exhibit weak van der Waals-type interactions. With increasing pressure, hydrogen bonding develops and strengthens between neighboring H2S molecules, reflected in a pronounced drop in S-H vibrational stretching frequency and also observed in first-principles calculations. At 17 GPa, an ordering process occurs where H2S molecules orient themselves to maximize hydrogen bonding and H-2 molecules simultaneously occupy a chemically distinct lattice site. Intermolecular forces in the H2S + H-2 system may be tuned with pressure from the weak hydrogen-bonding limit to the ordered hydrogen-bonding regime, resulting in a novel clathrate structure stabilized by cooperative interactions.