Phonon and elastic instabilities in rocksalt alkali hydrides under pressure: First-principles study

Abstract

The lattice and elastic instabilities of rocksalt (RS) alkali hydrides (LiH, NaH, KH, RbH, and CsH) under pressure are extensively studied to reveal the physically driven mechanism of the phase transition from RS to CsCl structure by using the pseudopotential plane-wave method within density functional theory. A universal pressure-induced soft transverse acoustic (TA) phonon mode is identified at the zone boundary $X$ point in the Brillouin zone for these compounds, signifying a structural instability. A predicted charge transfer from alkali to hydrogen with pressure might be attributable to the phonon softening. Moreover, a softening behavior in ${\mathrm{C}}_{44}$ shear modulus with pressure is predicted for NaH, KH, RbH, and CsH, while it is absent for LiH. Analysis of the calculated results suggested that with increasing pressure the predicted TA phonon softening behaviors, instead of ${\mathrm{C}}_{44}$ shear modulus instability, is mainly responsible for the pressure-induced structural phase transition. Furthermore, the current phonon calculations suggest that there might exist a similar $\mathrm{RS}\ensuremath{\rightarrow}\mathrm{Cs}\mathrm{Cl}$ phase transition in LiH.