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, CeH₂, CeH_2.5, and CeH₃. Under pressure, the experimental CaF₂-type structure of CeH₂ is likely to disproportionate to face-centered cubic (fcc) Ce and a cubic Pm3̅n (β-UH₃ type) structure of CeH₃ above 6 GPa. At further increasing pressures, fcc Ce will transform to a tetragonal I4/mmm structure above 12 GPa, while CeH₃ moves through the following sequence of phases: Pm3®n (β-UH₃ type) → Pm3̅n (A15 type) → R3®m; the corresponding transition pressures are calculated to be 10 and 70 GPa, respectively. The tetragonal I4₁/amd structure of CeH_2.5 has the similar decomposition as that of CeH₂. Finding this previously unreported pressure-induced decomposition of CeH₂ will pave the way for investigations on the nature of hydrogen-metal interactions.