High‐pressure lattice dynamics and thermodynamics in BaO

Abstract

AbstractUsing density functional theory and density functional perturbation theory we have studied lattice dynamics, dielectric and thermodynamic properties of the rocksalt (NaCl) phase of BaO under hydrostatic pressure, a material which has lately aroused many interests. Studying the lattice dynamics of the rocksalt phase at various pressures, in the range of the phase stability, we have found a lattice dynamical instability which governs the phase transition to NiAs phase. Monitoring the behaviour of the found soft mode, we have calculated the transition pressure of 10 GPa in an excellent agreement with experimental data. Lattice dynamics calculations reveal that the rocksalt structure is unstable with respect to soft transversal acoustic mode at a single point on the Brillouin zone boundary, which points to the fact that the transition is of displacive type. The observed pressure of the phase transition in structural measurements is reproduced more accurately than in previous calculations, which used enthalpy comparison method. Calculating the volume change during the phase transition from rocksalt to NiAs phase, we conclude that the phase transition might be of the second or weakly first order. Dielectric and thermodynamic properties at high pressures are also given. An anomalous thermal expansion coefficient is found up to about 10 K.