High-pressure phase transformations in CaH2

The structural stabilities, lattice dynamics and electronic properties of CaH2 under high pressures were studied using the density functional linear response theory. The calculations showed that CaH2 transforms from the PbCl2-type (Pnma) to the InNi2-type (P63/mmc) structure at ∼16 GPa in agreement with experiment. The theoretical and experimental Raman investigations demonstrated the occurrence of a high frequency E2g mode of the P63/mmc phase and explained why this mode was not observed in previous Raman experiments, in conflict with the proposed P63/mmc symmetry of the phase structure. The calculations of the phonon dispersion curves revealed the dynamical instability of the P63/mmc phase at pressures below 10 GPa caused by a soft transverse acoustic phonon mode at the zone boundary M point. The P63/mmc phase was predicted to be an insulator and to transform to a metallic phase with an AlB2-type structure (P6/mmm) at ∼138 GPa. According to the electron–phonon coupling calculation, the superconducting temperature of the P6/mmm CaH2 phase cannot be higher than 1 mK. The charge transfers in the Pnma, P63/mmc, and P6/mmm phases were also calculated and discussed.