Ab initio study of phase stability, lattice dynamics and thermodynamic properties of magnesium chalcogenides

The present work reports a comprehensive study of structural, dielectric, lattice dynamic and thermodynamic properties of magnesium chalcogenides MgX (X = S, Se, and Te) in rock-salt, zinc-blende and wurtzite crystal structures. Density functional theory calculations were performed using Perdew-Burke-Ernzerhof (PBE) functional (hereafter, PBE) and along with van der Walls interactions (hereafter, DFT-D). Our calculations show that the inclusion of van der Walls interactions improves the results and predict structural parameters close to the experimental values than using PBE functional alone. Both approaches show that the rock-salt phase is the ground state stable phase of MgS. The DFT-D calculations indicate that rock-salt whereas PBE results show wurtzite as the stable crystal structure for MgSe and MgTe respectively. Further, density functional perturbation theory has been employed to obtain the phonon dispersion curves and phonon density of states. The present phonon calculations show that these compounds are dynamically stable in the three considered phases. Thereafter, temperature dependent heat capacity at constant volume and entropy are also presented and discussed.