Development of high pressure techniques above 100 GPa and their application to materials in the core

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1 Structural phase transition of solid hydrogen and metal hydrides Optical studies have demonstrated that solid hydrogen exhibits three crystalline phases. Orientationaly disordered phase-I (hcp structure) transformed through phase-II into phase-III, which is considered to be an ordered phase. The transition boundary between phase-I and phase-II, which is reported to exhibit a strong isotope effect, shifts to lower pressure for deuterium. Solid n-D_2 transforms into phase-II around 28 GPa at 15 K. X-ray powder diffraction experiments at 15 K were performed at pressure up to 57 GPa. The results suggest that the center of each hydrogen molecule in phase-II is still on the hcp lattice point. The pressure-volume relation for deuterium at 15 K was also measured, and the bulk modulus and its pressure derivative are determined as 0.386 GPa and 6.43, respectively. Powder X-ray diffraction experiments of Re-hydrogen system were performed at high pressures and room temperature. Under pressure ab … More ove 5 GPa, rhenium metal (hcp) incorporated hydrogen in octahedral interstices of an hcp lattice. Superstructure reflection lines, which were produced by the displacements of rhenium atoms from the hcp lattice points, were observed. We conclude that hydrogen is incorporated in every second layer along the c-axis of an hcp lattice, which results in an anti-CdI_2 type structure.2 Pressure scale by the use of a diamond anvil When a diamond anvil cell is used for pressure generation, the ruby gauge that the pressure is determined from the wavelength shift of the fluorescence line of a ruby by pressure is commonly used. Above 100 GPa, however, the equation of state of gold or platinum is used for pressure determination because the fluorescence weakens abruptly. In this case, the unit cell volume of metals cannot be determined without synchrotron radiation sources at such high pressures. We measured the Raman shift of the diamond used as a anvil up to 220 GPa and proposed a new pressure scale by the use of a diamond anvil. Less