Structures and phase transitions of ScH₃ under high pressure

The structures and the phase transitions of ScH₃ under high pressure are investigated using first-principles calculations. The calculated structural parameters at zero pressure agree well with the available experimental data. With increasing pressure, the transition sequence hcp (GdH₃-type)→C2/m→fcc→hcp (YH₃-type)→Cmcm of ScH₃ is predicted first; the corresponding transition pressures at 0 K are 23 GPa, 25 GPa, 348 GPa, and 477 GPa, respectively. The C2/m symmetry structure is a possible candidate but not a good one as the intermediate state from hexagonal to cubic in ScH₃. On the other hand, via the analysis of the structures of hexagonal ScH_2.9, cubic ScH₃, and cubic ScH₂, we find that the repulsive interactions of H-H atoms must play an important role in the transition from hexagonal to cubic.