Structural, elastic, electronic and dynamical properties of Ba₂MgWO₆ double perovskite under pressure from first principles

Abstract

Ab initio calculations within the framework of density-functional theory employing the local density approximation have been performed to study the structural, elastic, electronic and dynamical properties for cubic double perovskite Ba₂MgWO₆ under hydrostatic pressure. The calculated ground-state properties and compression curve are in good agreement with the available experimental results. Pressure-induced enhancements of elastic constants, aggregate elastic moduli, elastic wave velocities and Debye temperature are observed, without any softening behaviors. Upon compression, the fundamental indirect energy gap E _g ^Γ−X first increases slightly and then monotonically decreases. A linear-response approach is adopted to derive the full phonon-dispersion curves and phonon density of states. Evolution with pressure of the zone-center phonon frequencies for Raman- and infrared-active modes is analyzed. A pressure-induced soft optically silent T _1g phonon mode is identified near the Γ point, signifying a structural dynamical instability. Our calculated results reveal that, when the pressure is high enough, besides bond shortening, the W-O-Mg bond becomes nonlinear, resulting in octahedral tilting distortion and thus a slight departure from the ideal cubic symmetry.