Theoretical Study of Phase Separation of Scandium Hydrides under High Pressure

Abstract

We report theoretical calculations of the static ground-state structures and pressure-induced phase transformations of three scandium hydrides: ScH, ScH2, and ScH3. For the monohydride, ScH, we predict several phases to be more stable at 1 atm than the previously suggested rock-salt structure, in particular one of P42/mmc symmetry. The NaCl-type structure for ScH takes over at 10 GPa and dominates over a wide pressure range until it is replaced by a Cmcm structure around 265 GPa. Under pressure, the experimental P = 1 atm CaF2-type structure of ScH2 should transform to a C2/m structure around 65 GPa, which then is likely to disproportionate to NaCl-type ScH and face-centered cubic ScH3 above 72 GPa. According to theory, as the pressure is elevated, ScH3 moves through the following sequence of phases: P63 → Fm3̅m → P63/mmc(YH3-type) → Cmcm; the corresponding transition pressures are calculated to be 29, 360, and 483 GPa, respectively. The predicted disproportionation tendencies of ScH2 are fascinating: stable to decomposition to ScH and ScH3 at low pressures, it should begin to disproportionate near 72 GPa. However, the process is predicted to reverse at still higher pressures (above 300 GPa). We also find ScH to be stable to disproportionation to Sc and ScH2 above ∼25 GPa. The three hydrides are metallic, except for (at low pressures) ScH3.