Long-period stacking structures in yttrium trihydride at high pressure

Machida, Akihiko; Omura, Ayako*; Watanuki, Tetsu; Aoki, Katsutoshi; Takemura, Kenichi*

The pressure induced structural transition of yttrium tri-hydride YH has been investigated by X-ray diffraction experiments under hydrostatic pressure at room temperature. The hexagonal metal lattice transforms to a face-centered cubic (fcc) one through an intermediate state, which appears between 12-22 GPa. The diffraction patterns of the intermediate state are interpreted in terms of long-period rhombohedral polytypes with the periodic arrangements of hexagonal-type () and fcc-type () stacking layers of the yttrium metals. These polytypes gradually transforms toward the fcc metal lattice with a successive increase in the fcc-type component upon compression. The hydrogen-hydrogen repulsive interactions and hydrogen-metal bonding likely play a dominant role in the gradual hexagonal-fcc lattice conversion via the long-period polytypic structures.