First principles calculation of phonon dispersion, thermodynamic properties andB 1-to-B 2 phase transition of lighter alkali hydrides

Abstract

The phonon dispersions of LiD, LiH and NaH forB 1 and B 2 phases are computed using density-functional perturbation theory(DFPT) with both local density (LDA) and generalized gradient (GGA)approximations. It is found from the phonon dispersion curves that theB 2 phase is unstable at low pressure for all the systems considered. From the vibrational freeenergy, the coefficient of the linear thermal expansion, the heat capacity and the vibrationalentropy as a function of temperature at zero pressure are calculated within the frameworkof the quasiharmonic approximation. Very good agreement is found for these propertiesexcept in the case of the GGA at high temperature. The equation of states for NaHB 1 andB 2 phases at300 K and the B 1-to-B 2 phase transition pressure are in excellent agreement with experimental results. The equation of state forthe LiH B 1 phase agrees well with experiments and recent theoretical calculations. The estimatedB 1-to-B 2 phase transition pressure (308 GPa) is also in good agreement with other theoreticalcalculations.