Ab initio investigation of CaO-ZnO alloys under high pressure

Ca(x)Zn(1–x)O alloys are potential candidates to achieve wide band-gap, which might significantly promote the band gap engineering and heterojunction design. We performed a crystal structure search for CaO-ZnO system under pressure, using an ab initio evolutionary algorithm implemented in the USPEX code. Four stable ordered Ca(x)Zn(1–x)O structures are found in the pressure range of 8.7–60 GPa. We further constructed the pressure vs. composition phase diagram of CaO-ZnO alloys based on the detailed enthalpy calculations. With the increase in Ca concentration, the CaO-ZnO alloy first undergoes a hexagonal to monoclinic transition, and then transforms back to a hexagonal phase. At Above 9 GPa, there is no cubic structure in the alloys, in contrast to the insostructural components (B1-B1). The band gap of the Ca(x)Zn(1–x)O alloy shows an almost linear increase as a function of the Ca concentration. We also investigated the variation regularity of the band gap under pressure.