Theoretical calculations and an assessment of recent experimental results for dense solid hydrogen lead to a unique scenario for the metallization of hydrogen under pressure. The existence of layered structures based on graphene sheets gives rise to an electronic structure related to unique features found in graphene that are well studied in the carbon phase. The honeycombed layered structure for hydrogen at high density, first predicted in molecular calculations, produces a complex optical response. The metallization of hydrogen is very different from that originally proposed via a phase transition to a close-packed monoatomic structure, and different from simple metallization recently used to interpret recent experimental data. These different mechanisms for metallization have very different experimental signatures. We show that the shift of the main visible absorption edge does not constrain the point of band gap closure, in contrast with recent claims. This conclusion is confirmed by measured optical spectra, including spectra obtained to low photon energies in the infrared region for phases III and IV of hydrogen.

BaReH9 is an exceedingly high-hydrogen-content metal hydride that is predicted to exhibit interesting behavior under pressure. The high-pressure electronic properties of this material were investigated using diamond-anvil-cell electrical conductivity techniques to megabar (100 GPa) pressures. The measurements show that BeReH9 transforms into a metal and then a superconductor above 100 GPa with a maximum transition temperature (T-c) near 7 K. The occurrence of superconductivity was confirmed by the suppression of the resistance drop upon application of magnetic fields. The transition to the metallic phase is sluggish, but it is accelerated by laser irradiation. Raman scattering and X-ray diffraction measurements, used to supplement the electrical measurements, indicate that the Ba-Re sublattice is largely preserved upon compression under the conditions explored, but there is a possibility that hydrogen atoms are gradually disordered under pressure. This is suggested by the sharpening of the peaks in Raman spectroscopy and X-ray diffraction upon heat treatment, as well as the temperature dependence of the resistance under pressure. The data suggest that the transition to the superconducting state is first-order. The possibility that the transition is associated with the breakdown of BeReH9 is discussed.

Fe-S-P compounds have been observed in many meteorites and could be the important components in planetary cores. Here we investigated the phase stability of Fe-3(S,P) solid solutions and synthesized high-quality Fe3(Si_ 13x) high-pressure phases in the multi-anvil press. The physical properties of Fe-3(S0.5130.5) were further studied in the diamond-anvil cell by synchrotron X-ray diffraction and emission spectroscopy. The solubility of S in the Fe-3(S,P) solid solution increases with increasing pressure. The minimum pressure to synthesize the pure Fe3S and Fe-3(S0.13P0.87) is about 21 and 8 GPa, respectively. The observed discontinuity in unit-cell parameters at about 18 GPa is caused by the high-spin to low spin transition of iron, supported by X-ray emission spectroscopy data. The sulfur solubility in Fe-3(S,P) solid solutions could be an excellent pressure indicator if such solid solutions are found in nature.

Raman spectroscopy of dense hydrogen and deuterium performed to 325 GPa at 300 K reveals previously unidentified transitions. Detailed analysis of the spectra from multiple experimental runs, together with comparison with previous infrared and Raman measurements, provides information on structural modifications of hydrogen as a function of density through the I-III-IV transition sequence, beginning near 200 GPa at 300 K. The data suggest that the transition sequence at these temperatures proceeds by formation of disordered stacking of molecular and distorted layers. Weaker spectral changes are observed at 250, 285, and 300 GPa, that are characterized by discontinuities in pressure shifts of Raman frequencies, and changes in intensities and linewidths. The results indicate changes in structure and bonding, molecular orientational order, and electronic structure of dense hydrogen at these conditions. The data suggest the existence of new phases, either variations of phase IV, or altogether new structures.

The discovery of diamonds and highly reduced minerals in podiform chromitites, which have generally been interpreted as magmatic rocks formed from partial melts of upper mantle peridotites under low-pressure conditions, has raised many questions about the origin of these enigmatic bodies. In order to provide experimental constraints on the formation and emplacement of podiform chromititesin ophiolites, we carried out a number of multi-anvil experiments in the magnesiochromite + SiO2 system at temperatures of 1000-1600 degrees C and pressures of 5-15 GPa. The experimental results demonstrate that magnesiochromite is stable up to 14 GPa and decomposed into eskolaite (Cr2O3) together with a quenchable modified ludwigite-structured phase [(Fe, Mg)(2)(Al, Cr)(2)O-5] at higher pressures, thus placing an approximate maximum depth for chromite crystallization and/or metamorphism. This depth corresponds to the top of the mantle transition zone (MTZ) at 410 km. The ludwigite-structured post-chromite phase has significant implications for understanding phase transformations and Cr incorporation/partitioning of minerals in the MTZ. On the basis of our results, we propose a multi-stage model for the formation of podiform chromitites that incorporates the geochemical, textural and mineralogical features of these bodies. (C) 2015 Elsevier B.V. All rights reserved.

Inside the cages of hypothetical carbon clathrates there is precious little room, even for the smallest atoms, such as Li-unless it is the Li+ ion that is inserted, in which case a compensating negative charge should be distributed over the carbon cage. The hypothesis explored in this paper is that Li insertion can be achieved with appropriate B substitution within the framework. The resulting structures of 2Li@C10B2 (Clathrate VII), 8Li@C38B8 (Clathrate I), 7Li@C33B7 (Clathrate IV), 6Li@C28B6 (Clathrate H), and 6Li@C28B6 (Clathrate II) are definitely stabilized in theoretical calculations, especially under elevated pressure, as judged by enthalpy criteria and bond length metrics. Different strategies for B substitution (symmetry reduction, following the parent charge distribution, and substitution on the most weakened bonds, relieving stress on bond angles) are explored. Two possible competing channels for Li doping B substitution, formation of LiBC and C-vacancies, are investigated.

We have computed the correlated electronic structure of FeSe and its dependence on the A(1g) mode versus compression. Using the self-consistent density functional theory-dynamical mean field theory (DFT-DMFT) with continuous time quantum Monte Carlo, we find that there is greatly enhanced coupling between some correlated electron states and theA(1g) lattice distortion. Superconductivity in FeSe shows a very strong sensitivity to pressure, with an increase in T-c of almost a factor of 5 within a few GPa, followed by a drop, despite monotonic pressure dependence of almost all electronic properties. We find that the maximum A(1g) deformation potential behaves similar to the experimental T-c. In contrast, the maximum deformation potential in DFT for this mode increases monotonically with increasing pressure.

We have investigated the high-pressure behavior of Fe3O4 by in situ X-ray diffraction measurements from 11 to 103 GPa. Up to 70 GPa, the previous observed high-pressure Fe3O4 phase (h-Fe3O4) is stable, with a CaTi2O4-type structure. The compression curve shows an abnormal volume contraction at about 50 GPa, likely associated with the magnetic moment collapse observed at that pressure. Fitting the compression data up to 45 GPa to the Birch-Murnaghan equation of state yields a bulk modulus, K-T0 = 172 GPa, and V-0 = 277 angstrom(3), with fixed K' = 4. At a pressure between 64 and 73 GPa, a new structural transition was observed in Fe3O4, which can be attributed to a martensitic transformation as described by Yamanaka et al. (2008) for post-spinel structural transition. The diffraction data can be best fitted with a Pnma space group. No breakdown of Fe3O4 was observed up to at least 103 GPa. The new high-pressure polymorph is about 6% denser than the h-Fe3O4 phase at 75 GPa.

Highly porous nanostructures with large surface areas are typically employed for electrical double-layer capacitors to improve gravimetric energy storage capacity; however, high surface area carbon-based electrodes result in poor volumetric capacitance because of the low packing density of porous materials. Here, we demonstrate ultrahigh volumetric capacitance of 521 F cm(-3) in aqueous electrolytes for non-porous carbon microsphere electrodes co-doped with fluorine and nitrogen synthesized by low-temperature solvothermal route, rivaling expensive RuO2 or MnO2 pseudo-capacitors. The new electrodes also exhibit excellent cyclic stability without capacitance loss after 10,000 cycles in both acidic and basic electrolytes at a high charge current of 5 Ag-1. This work provides a new approach for designing high-performance electrodes with exceptional volumetric capacitance with high mass loadings and charge rates for long-lived electrochemical energy storage systems.

Recent studies reveal a pressure induced transition from a paramagnetic tetragonal phase (T) to a collapsed tetragonal phase (CT) in CaFe2As2, which was found to be superconducting with pressure at low temperature. We have investigated the effects of electron correlation and a local fluctuating moment in both tetragonal and collapsed tetragonal phases of the paramagnetic CaFe2As2 using self-consistent DFT-DMFT with continuous time quantum Monte Carlo as the impurity solver. From the computed optical conductivity, we find a gain in the optical kinetic energy due to the loss in Hund's rule coupling energy in the CT phase. We find that the transition from T to CT turns CaFe2As2 from a bad metal into a good metal. Computed mass enhancement and local moments also show a significant decrease in the CT phase, which confirms the suppression of the electron correlation in the CT phase of CaFe2As2.