Structural stability and thermal properties of BeO from the quasiharmonic approximation

Abstract

The phase diagram and thermal properties of BeO crystal are calculated within thequasiharmonic approximation and using density functional theory. The wurtzite, zinc blendeand 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. Theresulting free energy provides predictions for the temperature dependence of variousquantities 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 phasesolely. The zinc blende structure is energetically not preferred in the investigatedrange of pressures and temperatures. Theoretical results are compared with theavailable experimental data and other ab initio calculations. Existing discrepanciesbetween quasiharmonic theory and experiment are discussed and some explanation isgiven.