Ab initio lattice dynamics evidence for the broken-symmetry phase of solid hydrogen

Abstract

The lattice dynamics of solid hydrogen in the broken-symmetry phase (phase II) areextensively studied using density functional linear-response theory. The full phonondispersion curves in the whole Brillouin zone for the previously proposed candidatestructures are examined for the first time. It is found that the energetically preferredPca 21 structure shows exclusively dynamical stability in the pressure range ofphase II (110–150 GPa), as indicated by the absence of phonon softening.A pressure-induced soft transverse acoustic (TA) phonon mode is identifiedand the TA mode completely softens at the zone boundary of the Y point at∼151 GPa, which coincides well with the experimentally observed transition pressure(150 GPa) from phase II to phase III. Moreover, the analysis of the eigenvector ofthe TA soft mode suggested that this phonon softening will result in enlargedintramolecular bond lengths. This fact might serve to explain the experimentalobservation of a sudden decrease in Raman and IR active vibron frequencies during thephase II to phase III transition. The physical reason for the reduction of Ramanvibron frequency with pressure has been discussed. The newly proposed structure with Pa3-type local order has been explored for the first time. However, the calculated results oftotal energy, band structure, and phonon do not support the choice of structure.