Synchrotron x-ray diffraction and Raman spectroscopy have been used to study the chemical reactions of molecular hydrogen (H-2) with sulfur (S) at high pressures. We find theoretically predicted Cccm and Im (3) over barm H3S to be the reaction products at 50 and 140 GPa, respectively. Im (3) over barm H3S is a stable crystalline phase above 140 GPa and it transforms to R3mH(3)S on pressure release below 140 GPa. The latter phase is (meta) stable down to at least 70 GPa where it transforms to Cccm H3S upon annealing (T < 1300 K) to overcome the kinetic hindrance. Cccm H3S has an extended structure with symmetric hydrogen bonds at 50 GPa, and upon decompression it experiences a transformation to a molecular mixed H2S-H-2 structure below 40 GPa without any apparent change in the crystal symmetry.

The elasticity at high pressure of solid hydrogen in hexagonal close-packed (hcp) phase I has been examined experimentally by laser acoustics technique in a diamond anvil cell, up to 55 GPa at 296 K, and theoretically using pair and three-body semiempirical potentials, up to 160 GPa. In the experiments on H-2 and D-2, the compressional sound velocity has been measured; the Poisson's ratio has been determined by combining these data with the previously reported equation of state. At room temperature, the difference between the adiabatic and isothermal processes vanishes above 25 GPa but cannot be neglected at lower pressure. Theoretically, all five elastic constants of hcp hydrogen have been calculated, and various derived elastic quantities are presented. The elastic anisotropy of hcp hydrogen was found to be significant, with Delta P approximate to 1.2, Delta S-1 Delta approximate to 1.7, and Delta S-2 approximate to 1. Calculations suggest the Poisson's ratio to decrease with pressure reaching a minimum value of 0.28 at 145 GPa. In the experiment, the Poisson's ratio is also found to decrease with pressure. Theoretical calculations show that the inclusion of zero-point vibrations on the elastic properties of H-2 does not result in any drastic changes of the behavior of the elastic quantities.

A combined theoretical and experimental study of lithium palladium deuteride (Li2PdD2) subjected to pressures up to 50 GPa reveals one structural phase transition near 10 GPa, detected by synchrotron powder x-ray diffraction, and metadynamics simulations. The ambient-pressure tetragonal phase of Li2PdD2 transforms into a monoclinic C2/m phase that is distinct from all known structures of alkali metal-transition metal hydrides/deuterides. The structure of the high-pressure phase was characterized using ab initio computational techniques and from refinement of the powder x-ray diffraction data. In the high-pressure phase, the PdD2 complexes lose molecular integrity and are fused to extended [PdD2](infinity) chains. The discovered phase transition and new structure are relevant to the possible hydrogen storage application of Li2PdD2 and alkali metal-transition metal hydrides in general. Published by AIP Publishing.

Helium is generally understood to be chemically inert and this is due to its extremely stable closed-shell electronic configuration, zero electron affinity and an unsurpassed ionization potential. It is not known to form thermodynamically stable compounds, except a few inclusion compounds. Here, using the ab initio evolutionary algorithm USPEX and subsequent high-pressure synthesis in a diamond anvil cell, we report the discovery of a thermodynamically stable compound of helium and sodium, Na2He, which has a fluorite-type structure and is stable at pressures >113 GPa. We show that the presence of He atoms causes strong electron localization and makes this material insulating. This phase is an electride, with electron pairs localized in interstices, forming eight-centre two-electron bonds within empty Na-8 cubes. We also predict the existence of Na2HeO with a similar structure at pressures above 15 GPa.

"Chemical precompression" through introducing impurity atoms into hydrogen has been proposed as a method to facilitate metallization of hydrogen under external pressure. Here we selected Ar(H-2)(2), a hydrogen-rich compound with molecular hydrogen, to explore the effect of "doping" on the intermolecular interaction of H-2 molecules and metallization at ultrahigh pressure. Ar(H-2)(2) was studied experimentally by synchrotron X-ray diffraction to 265 GPa, by Raman and optical absorption spectroscopy to 358 GPa, and theoretically using the density-functional theory. Our measurements of the optical bandgap and the vibron frequency show that Ar(H-2)(2) retains 2-eV bandgap and H-2 molecular units up to 358 GPa. This is attributed to reduced intermolecular interactions between H-2 molecules in Ar(H-2)(2) compared with that in solid H-2. A splitting of the molecular vibron mode above 216 GPa suggests an orientational ordering transition, which is not accompanied by a change in lattice symmetry. The experimental and theoretical equations of state of Ar(H-2)(2) provide direct insight into the structure and bonding of this hydrogen-rich system, suggesting a negative chemical pressure on H-2 molecules brought about by doping of Ar.

Dias and Silvera (Research Article, 17 February 2017, p. 715) report on the observation of the Wigner-Huntington transition to metallic hydrogen at 495 gigapascals at 5.5 and 83 kelvin. Here, we show that the claim of metallic behavior is not supported by the presented data, which are scarce, contradictory, and do not prove the presence of hydrogen in the high-pressure cavity.

The exceptional ability of carbon to form sp(2) and sp(3) bonding states leads to a great structural and chemical diversity of carbon-bearing phases at nonambient conditions. Here we use laser-heated diamond-anvil cells combined with synchrotron x-ray diffraction, Raman spectroscopy, and first-principles calculations to explore phase transitions in CaCO3 at P > 40 GPa. We find that postaragonite CaCO3 transforms to the previously predicted P2(1)/c CaCO3 with sp(3)-hybridized carbon at 105 GPa (similar to 30 GPa higher than the theoretically predicted crossover pressure). The lowest-enthalpy transition path to P2(1)/c CaCO3 includes reoccurring sp(2) and sp3 CaCO3 intermediate phases and transition states, as revealed by our variable-cell nudged-elastic-band simulation. Raman spectra of P2(1)/c CaCO3 show an intense band at 1025 cm(-1), which we assign to the symmetric -O stretching vibration based on empirical and first-principles calculations. This Raman band has a frequency that is similar to 20% lower than the symmetric C-O stretching in sp(2) CaCO3 due to the C-O bond length increase across the sp(2)-sp(3) transition and can be used as a fingerprint of tetrahedrally coordinated carbon in other carbonates.

The iron spin transition directly affects properties of lower mantle minerals and can thus alter geophysical and geochemical characteristics of the deep Earth. While the spin transition in ferropericlase has been documented at P similar to 60GPa and 300K, experimental evidence for spin transitions in other rock-forming minerals, such as bridgmanite and post-perovskite, remains controversial. Multiple valence, spin, and coordination states of iron in bridgmanite and post-perovskite are difficult to resolve with conventional spin probing techniques. Optical spectroscopy, on the other hand, can discriminate between high and low spin and between ferrous and ferric iron at different sites. Here we establish the optical signature of low spin Fe3+O6, a plausible low spin unit in bridgmanite and post-perovskite, by optical absorption experiments in diamond anvil cells. We show that the optical absorption of Fe3+O6 in new aluminous phase (NAL) is very sensitive to the iron spin state and may represent a model behavior of bridgmanite and post-perovskite across the spin transition. Specifically, an absorption band centered at similar to 19,000cm(-1) is characteristic of the (T2gT1g)-T-2-T-2 ((2)A(2g)) transition in low spin Fe3+ in NAL at 40GPa, constraining the crystal field splitting energy of low spin Fe3+ to similar to 22,200cm(-1), which we independently confirm by first-principles calculations. Together with available information on the electronic structure of Fe3+O6 compounds, we show that the spin-pairing energy of Fe3+ in an octahedral field is similar to 20,000-23,000cm(-1). This implies that octahedrally coordinated Fe3+ in bridgmanite is low spin at P>similar to 40GPa.

The importance for the global carbon cycle, the P-T phase diagram of CaCO3 has been under extensive investigation since the invention of the high-pressure techniques. However, this study is far from being completed. In the present work, we show the existence of two new high-pressure polymorphs of CaCO3. The crystal structure prediction performed here reveals a new polymorph corresponding to distorted aragonite structure and named aragonite-II. In situ diamond anvil cell experiments confirm the presence of aragonite-II at 35 GPa and allow identification of another high-pressure polymorph at 50 GPa, named CaCO3-VII. CaCO3-VII is a structural analogue of CaCO3-P2(1)/c-1, predicted theoretically earlier. The P-T phase diagram obtained based on a quasi-harmonic approximation shows the stability field of CaCO3-VII and aragonite-II at 30-50 GPa and 0-1200 K. Synthesized earlier in experiments on cold compression of calcite, CaCO3-VI was found to be metastable in the whole pressure temperature range.

Dichalcogenides are known to exhibit layered solid phases, at ambient and high pressures, where 2D layers of chemically bonded formula units are held together by van derWaals forces. These materials are of great interest for solid-state sciences and technology, along with other 2D systems such as graphene and phosphorene. SiS2 is an archetypal model system of the most fundamental interest within this ensemble. Recently, high pressure (GPa) phases with Si in octahedral coordination by S have been theoretically predicted and also experimentally found to occur in this compound. At variance with stishovite in SiO2, which is a 3D network of SiO6 octahedra, the phases with octahedral coordination in SiS2 are 2D layered. Very importantly, this type of semiconducting material was theoretically predicted to exhibit continuous bandgap closing with pressure to a poor metallic state at tens of GPa. We synthesized layered SiS2 with octahedral coordination in a diamond anvil cell at 7.5-9 GPa, by laser heating together elemental S and Si at 1300-1700 K. Indeed, Raman spectroscopy up to 64.4 GPa is compatible with continuous bandgap closing in this material with the onset of either weak metallicity or of a narrow bandgap semiconductor state with a large density of defect-induced, intra-gap energy levels, at about 57 GPa. Importantly, our investigation adds up to the fundamental knowledge of layered dichalcogenides. Published by AIP Publishing.