Phase separation of lanthanum hydride under high pressure

Machida, Akihiko; Watanuki, Tetsu; Kawana, Daichi; Aoki, Katsutoshi

Structural change of lanthanum dihydride, which has an fcc metal lattice with tetrahedral interstitial sites fully occupied with hydrogen atoms and empty octahedral sites, has been investigated at high pressures up to 20 GPa at ambient temperature by synchrotron radiation X-ray diffraction. The coexistence state of two fcc metal lattices observed above 11 GPa is interpreted in terms of phase separation from the dihydride toward a solid solution and trihydride states, in both of which the octahedral interstitial sites are partially occupied with hydrogen atoms. A gradual distortion from the cubic to a tetragonal lattice is observed prior to the phase separation. The coexistence phase goes back to the dihydride fcc phase via the lattice distorted phase with decreasing pressure.