Abstract
The structural stabilities, lattice dynamics and electronic properties ofCaH2 underhigh pressures were studied using the density functional linear response theory. The calculations showed thatCaH2 transformsfrom the PbCl2-type (Pnma)to the InNi2-type (P63/mmc)structure at ∼16 GPa in agreement with experiment. The theoretical and experimentalRaman investigations demonstrated the occurrence of a high frequencyE2g modeof the P63/mmc phase and explained why this mode was not observed in previous Raman experiments, in conflict with theproposed P63/mmc symmetry of the phase structure. The calculations of the phonondispersion curves revealed the dynamical instability of theP63/mmc phase atpressures 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 anAlB2-typestructure (P6/mmm) at ∼138 GPa. According to the electron–phonon coupling calculation, the superconducting temperature ofthe P6/mmm CaH2 phase cannot be higher than 1 mK. The charge transfers in thePnma,P63/mmc,and P6/mmm phases were also calculated and discussed.
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