New Insights into Phase Separation of Cerium Hydrides under Pressure.

We report theoretical calculations of the static ground-state structures and pressure-induced phase transformations of four cerium hydrides: CeH, CeH 2 , CeH 2.5 , and CeH 3 . Under pressure, the experimental CaF 2 -type structure of CeH 2 is likely to disproportionate to face-centered cubic (fcc) Ce and a cubic Pm 3̅ n (β-UH 3 type) structure of CeH 3 above 6 GPa. At further increasing pressures, fcc Ce will transform to a tetragonal I 4/ mmm structure above 12 GPa, while CeH 3 moves through the following sequence of phases: Pm 3 ®n (β-UH 3 type) → Pm 3̅ n (A15 type) → R 3 ®m ; the corresponding transition pressures are calculated to be 10 and 70 GPa, respectively. The tetragonal I 4 1 / amd structure of CeH 2.5 has the similar decomposition as that of CeH 2 . Finding this previously unreported pressure-induced decomposition of CeH 2 will pave the way for investigations on the nature of hydrogen-metal interactions.