Thermal properties of CaO and SrO and their B1-B2 phase boundaries: Validity of the quasi-harmonic approximation

In this paper, we will further confirm that the validity range of the quasi-harmonic approximation (QHA) can be significantly extended by explicitly calculating the Helmholtz free energy over an appropriate volume range. As in the case of MgO, we performed accurate QHA calculations of the thermal properties of the B1 phase of CaO and SrO over a wide temperature range, as well as their B1-B2 phase boundaries in the pressure- temperature space. This finding contradicts a recent conclusion that the QHA is inadequate for CaO, and it is therefore expected to fail for other alkaline-earth oxides containing more polarizable cations. Moreover, the validity range of the QHA is found to be consistent with the bond ionicity in MgO, CaO and SrO.

Automated summary

In this study, it is further confirmed that the validity range of the quasi-harmonic approximation (QHA) can be extended by explicitly calculating the Helmholtz free energy over an appropriate volume range. Accurate QHA calculations were performed for the thermal properties of the B1 phase of CaO and SrO over a wide temperature range, as well as their B1-B2 phase boundaries in the pressure- temperature space. The validity range of the QHA is consistent with the bond ionicity in MgO, CaO and SrO.