First principles calculation of phonon dispersion, thermodynamic properties and B1-to-B2 phase transition of lighter alkali hydrides

The phonon dispersions of LiD, LiH and NaH for B1 and B2 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 the B2 phase is unstable at low pressure for all the systems considered. From the vibrational free energy, the coefficient of the linear thermal expansion, the heat capacity and the vibrational entropy as a function of temperature at zero pressure are calculated within the framework of the quasiharmonic approximation. Very good agreement is found for these properties except in the case of the GGA at high temperature. The equation of states for NaH B1 and B2 phases at 300 K and the B1-to-B2 phase transition pressure are in excellent agreement with experimental results. The equation of state for the LiH B1 phase agrees well with experiments and recent theoretical calculations. The estimated B1-to-B2 phase transition pressure (308 GPa) is also in good agreement with other theoretical calculations.