X-ray and Neutron Study on the Structure of Hydrous SiO2 Glass up to 10 GPa

The structure of hydrous amorphous SiO2 is fundamental in order to investigate the effects of water on the physicochemical properties of oxide glasses and magma. The hydrous SiO2 glass with 13 wt.% D2O was synthesized under high-pressure and high-temperature conditions and its structure was investigated by small angle X-ray scattering, X-ray diffraction, and neutron diffraction experiments at pressures of up to 10 GPa and room temperature. This hydrous glass is separated into two phases: a major phase rich in SiO2 and a minor phase rich in D2O molecules distributed as small domains with dimensions of less than 100 angstrom. Medium-range order of the hydrous glass shrinks compared to the anhydrous SiO2 glass by disruption of SiO4 linkage due to the formation of Si-OD deuterioxyl, while the response of its structure to pressure is almost the same as that of the anhydrous SiO2 glass. Most of D2O molecules are in the small domains and hardly penetrate into the void space in the ring consisting of SiO4 tetrahedra

Behavior of intermolecular interactions in alpha-glycine under high pressure

Pressure-response on the crystal structure of deuterated alpha-glycine was investigated at room temperature, using powder and single-crystal X-ray diffraction, and powder neutron diffraction measurements under high pressure. No phase change was observed up to 8.7 GPa, although anisotropy of the lattice compressibility was found. No significant changes in the compressibility and the intramolecular distance between non-deuterated alpha-glycine and deuterated alpha-glycine were observed. Neutron diffraction measurements indicated the distance of the intermolecular D center dot center dot center dot O bond along with the c-axis increased with compression up to 6.4 GPa. The distance of another D center dot center dot center dot O bond along with the a-axis decreased with increasing pressure and became the shortest intermolecular hydrogen bond above 3 GPa. In contrast, the lengths of the bifurcated N-D center dot center dot center dot O and C-D center dot center dot center dot O hydrogen bonds, which are formed between the layers of the alpha-glycine molecules along the b-axis, decreased significantly with increasing pressure. The decrease of the intermolecular distances resulted in the largest compressibility of the b-axis, compared to the other two axes. The Hirshfeld analysis suggested that the reduction of the void region size, rather than shrinkage of the strong N-D center dot center dot center dot O hydrogen bonds, occurred with compression. Published by AIP Publishing

Anomalous hydrogen dynamics of the ice VII–VIII transition revealed by high-pressure neutron diffraction

International audienceSignificanceWe report high-pressure neutron data which reveal a highly unusual behavior of the hydrogen dynamics in ice under high pressure: The transition rate from hydrogen-disordered ice VII to ordered ice VIII slows down dramatically up to 10 GPa but becomes more rapid again above. This finding is quantitatively explainable by a competition between the rotational and translational dynamics of water molecules. Our findings reply to a long-standing problem of unexplained anomalies of ice VII in the 10–15 GPa range reported by Raman spectroscopy, X-ray diffraction, and proton conductivity. Since ice VII and ice VIII are prototypical hydrogen-bonded materials with order–disorder transition, the pressure-induced cross-over in the hydrogen dynamics found in this study might be commonly observable in other materials.AbstractAbove 2 GPa the phase diagram of water simplifies considerably and exhibits only two solid phases up to 60 GPa, ice VII and ice VIII. The two phases are related to each other by hydrogen ordering, with the oxygen sublattice being essentially the same. Here we present neutron diffraction data to 15 GPa which reveal that the rate of hydrogen ordering at the ice VII–VIII transition decreases strongly with pressure to reach timescales of minutes at 10 GPa. Surprisingly, the ordering process becomes more rapid again upon further compression. We show that such an unusual change in transition rate can be explained by a slowing down of the rotational dynamics of water molecules with a simultaneous increase of translational motion of hydrogen under pressure, as previously suspected. The observed cross-over in the hydrogen dynamics in ice is likely the origin of various hitherto unexplained anomalies of ice VII in the 10–15 GPa range reported by Raman spectroscopy, X-ray diffraction, and proton conductivity

Developments of nano-polycrystalline diamond anvil cells for neutron diffraction experiments

International audienceDevelopments of nano-polycrystalline diamond anvil cells for neutron diffraction experiments A new high-pressure cell for neutron diffraction experiments using nano-polycrystalline anvils is presented. The cell design, off-line pressure generation tests and a gas-loading procedure for this cell are described. The performance is illustrated by powder neutron diffraction patterns of ice VII to ~82 GPa. We also demonstrate the feasibility of single crystal neutron diffraction experiments of Fe3O4 at ambient conditions using this cell and discuss the current limitation and future developments

Hexagonal Close-packed Iron Hydride behind the Conventional Phase Diagram

六方最密充填構造の鉄は幅広い温度圧力領域で安定に存在することが知られている一方で、鉄水素化物ではこれまでに知られている鉄-水素系の温度圧力相図上には六方最密充填構造は存在しない。水素濃度を0.6以下になるように制御した状態において、高温高圧下放射光その場X線回折およびその場中性子回折実験を実施し、六方最密充填構造の鉄水素化物が形成されることを明らかにした。水素は鉄の作る八面体サイトを部分的に、そしてランダムに占有しており、水素による体積膨張率は面心立方構造のものよりも大きいことがわかった