Induced changes in vibrational properties of NaH and KH under hydrostatic pressure

The pressure induced phase transition from the NaCl to CsCl structure in NaH and KH has been investigated by means of first-principles calculations, and density functional linear-response theory. A pressure-induced soft-acoustic phonon mode is identified at 30 GPa, and 7.5 GPa for NaH and KH respectively. Phonon calculations suggest that the pressure induced instabilities of the transverse acoustic modes at the [ε00], and [εε0] directions are responsible for the phase transition of NaH and KH. Furthermore charge density analysis shows that there is charge transfer from the alkali ion to hydrogen (i.e., Na→H, K→H) inducing B1–B2 phase transition.

► A pressure-induced soft-acoustic phonon mode is identified at 30 GPa and 7.5 GPa for NaH and KH respectively. ► The transverse acoustic modes at the [ε00], and [εε0] directions are responsible for the phase transition of NaH and KH. ► There is charge transfer from the alkali ion to hydrogen (i.e., Na→H, K→H) inducing B1–B2 phase transition.