Study on crystal structures and compressibilities of solid hydrogen and transition metal hydrides at high pressures and high temperatures

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(1)Solid hydrogen : At temperature-pressure range of 30K-395K and 6GPa-100GPa, X-ray diffraction experiments were performed. The diffraction peaks assigned by an hcp lattice were observed in that temperature-pressure range. Pressure dependences of molar volume of hydrogen at 30, 150, 300 and 395 K were obtained. At pressures above 20 GPa, the molar volume is scarcely dependent on temperature. Lattice constants and the c/a ratio of an hcp lattice were continuously decreased by increase in pressure across the I-II phase boundary.(2)Metal hydrides : Rhenium metal incorporated hydrogen in interstices of its hcp lattice and formed a metal hydride at pressures above 7GPa. Hydrogen is incorporated in every second layer along the c-axis of an hcp lattice, which results in an anti-CdI_2 structure. Tungsten metal also incorporated hydrogen at pressures above 25GPa. In the rhenium hydride the arrangements of a metal sublattice did not change, while the tungsten sublattice changed from a bcc to an … More hcp when hydrogen was incorporated in a tungsten metal. The incorporated hydrogen atom randomly occupied the octahedral hole of the hcp lattice. The compressibility and the reaction with hydrogen of Fe-10%Ni alloy, which is considered to be the principal constituent of the earth's core, were investigated. The alloy incorporated hydrogen at pressure above 6 GPa and transformed from a bcc to a dhcp structure. On the other hand, the pristine alloy transformed from a bcc to an hcp at 15GPa. These transformations are similar to those of pure Fe. The transition pressures, however, are higher in the case of the alloy.(3)Multimegabar and a pressure scale : The first-order Raman spectra of diamond anvils used in a gasketed high-pressure cell were measured at pressures up to 300GPa. The high-frequency edge of the Raman band is represented by a function of pressure in the sample chamber. The application of the pressure dependence for pressure determination up to multimegabars pressure region is proposed. Less