Ab initio calculation of vibrational and thermodynamic properties of SrX (S, Se, Te) in the B1 (NaCl) and B2 (CsCl) structures

Using density–functional perturbation theory, we have investigated the lattice dynamics of SrX (X = S, Se and Te) and their pressure dependence in both B1 (NaCl structure) and B2 (CsCl structure) phases. Static calculations predict the B1–B2 transition to occur at 17.7, 13.6 and 12.3 GPa for SrS, SrSe and SrTe respectively. The inclusion of zero-point vibrations lowers these values. The splitting varies little with pressure. The results compare well with available experiments. We have also used the calculated dispersions in conjunction with quasi-harmonic approximation to predict temperature and pressure dependence of various quantities such as the thermal expansion coefficient, the bulk modulus and the heat capacity. Our results for the thermal expansion coefficient show that it decreases with pressure and does not show any negative behaviour. The predicted transition pressure decreases with temperature.