Thermal Properties of Divalent Metal Oxides: CaO as a Prototype

Commonly employed quasiharmonic approximation (QHA) is inadequate to account for intrinsic anharmonism such as phonon-phonon interaction, vacancy contribution, etc. Though anharmonic contributions are important at high-T and low-P, complete ab initio calculations are scanty due largely to laborious computational requirements. Nevertheless, some simple semi-empirical schemes can be used effectively to incorporate the anharmonism. We have proposed a simple computational scheme to include the effect of vacancy directly into the description within the mean-field potential approach, which calculates vibrational free energy of ions. Validity of the scheme is verified by taking CaO as a test case. Equilibrium properties at (T,P) = (0,0) is obtained within the tight-binding second-moment approximation (TB-SMA), whose parameters were determined using ultrasoft plane-wave pseudopotential within generalized gradient approximation. Present findings for thermal expansion and high-T EOS clearly show perceptible improvement over the case when vacancy contribution was not included. Some related thermodynamic properties are also calculated and compared with the available experimental and theoretical data.