Structural stability and thermal properties of BeO from the quasiharmonic approximation.

The phase diagram and thermal properties of BeO crystal are calculated within the quasiharmonic approximation and using density functional theory. The wurtzite, zinc blende and rocksalt phases of BeO are considered. Phonons are calculated versus external pressure. The pressure dependence of frequencies for the zone-center phonon modes is analyzed. The resulting free energy provides predictions for the temperature dependence of various quantities such as the equilibrium volume, thermal expansivity and isobaric heat capacity. The mean-squared vibrations of BeO atoms are investigated as well. The calculated (p,T) diagram predicts a structural phase transition from the wurtzite to rocksalt phase solely. The zinc blende structure is energetically not preferred in the investigated range of pressures and temperatures. Theoretical results are compared with the available experimental data and other ab initio calculations. Existing discrepancies between quasiharmonic theory and experiment are discussed and some explanation is given.