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Omura, Ayako; Machida, Akihiko; Watanuki, Tetsu; Aoki, Katsutoshi; Nakano, Satoshi*; Takemura, Kenichi*
Proceedings of Joint 20th AIRAPT - 43rd EHPRG International Conference on High Pressure Science and Technology (CD-ROM), 3 Pages, 2005/06
Yttrium hydride shows a structural change from a cubic dihydride to a hexagonal trihydride by hydrogenation. This is accompanied by the metal-insulator transition. The theoretical calculation of yttrium hydride speculates that the hybridization between 4d-Y and 1s-H leads to the opening of a large energy gap. They also predict that a transition from insulator to metal occurs when the volume is reduced to about 85% of the equilibrium volume in yttrium trihydride. Recently, we performed an infrared spectroscopy of yttrium trihydride to investigate pressure-induced metallization under high pressure. The insulator-metal transition occurred with an abrupt disappearance of the optical gap of 
1 eV at 23 GPa. There is a possibility of the electronic transition because no structural changes occur above 20 GPa. The electronic transition can be attributed to the delocalization of the 1s electrons bound to the H+ core ions or the rearrangement of hydrogen atoms.
Utsumi, Wataru; Saito, Hiroyuki; Kaneko, Hiroshi*; Kiriyama, Koji*; Aoki, Katsutoshi
Proceedings of Joint 20th AIRAPT - 43rd EHPRG International Conference on High Pressure Science and Technology (CD-ROM), 5 Pages, 2005/06
We were successful in synthesizing single crystals of GaN (Gallium Nitride) by slow cooling of its congruent melt under high pressure. It was confiremed by in situ X-ray diffraction that applying high pressures above 6.0 GPa completely prevented the decomposition and allowed the congruent melt of GaN at 2220
C. Using a cubic-anvil-type large volume high-pressure apparatus and GaN powder as a starting material, single crystal growth was performed by decreasing temperature from 2400C at 6.5 GPa. The X-ray rocking curve of the recovered sample showed very narrow line-width smaller than 30 arcsec, suggesting its low dislocation density.
Ga
N and InN crystals under high pressuresSaito, Hiroyuki; Utsumi, Wataru; Kaneko, Hiroshi*; Aoki, Katsutoshi
Proceedings of Joint 20th AIRAPT - 43rd EHPRG International Conference on High Pressure Science and Technology (CD-ROM), 4 Pages, 2005/06
We have performed synthesis studies of various III-V nitrides crystals, key materials for optoelectronic and high-power/frequency devices, using a cubic-anvil-type large volume high-pressure apparatus combined with in situ X-ray diffraction. Polly-crystallines of AlGaN alloys covering a composition range of 0 
x 1 were synthesized by a solid-phase reaction under high pressure. In situ X-ray diffraction profiles were measured to observe the alloying process, which started at around 800C under 6.0 GPa. Single crystal of Al
Ga
N was also successfully obtained by slow cooling of its melt from 2400C at 6.5 GPa. For InN, its phase diagram was determined under high P-T conditions up to 20GPa and 2000C based on the in situ observations, which demonstrates that 19GPa and 1900C are needed for its congruent melting.