Structural phase transition and electronic structure of binary CaO and SrO under high pressure

We have investigated pressure-induced structural phase transition and electronic structure of binary alkaline-earth monoxides. From our static enthalpy calculations, CaO and SrO are found to undergo a common structural phase transition from B1 to B2 at 59.2 GPa and 32.4 GPa, respectively. B33 phase is found to be meta-stable for both investigated compounds. We have calculated electronic structure with Tran-Blaha modified Becke-Johnson potential (TB-mBJ) potential and the obtained band gaps are improved over standard PBE-GGA functional. CaO and SrO compounds are found to be indirect band gap insulators at ambient pressure. Interestingly, electronic structure calculations at high pressure suggest that B2 phase of CaO is an indirect band gap semiconductor while B2 phase of SrO is a direct band gap semiconductor, i.e., pressure-induced indirect → direct band gap transition is observed in SrO.