Effects of magnesium substitution on electronic properties of calcium difluoride

Abstract

We use density functional theory to find the phase transitions of CaF2. The results show that CaF2 adopts the fluorite structure, with the space group Fm-3m, in the range 0f 0-8 GPa and transforms into the cotunnite structure, with the space group Pnma, above 8 GPa. The electronic band structure of the Pnma-CaF2 structure expresses an insulating phase, and the bandgap collapses under pressure above 70 GPa. In this work, we proposed a new decoration of Ca/Mg-substituted fluorides MgxCa1-xF2, using the cluster expansion technique which is commonly used to study alloys. We found that the Mg0.25Ca0.75F2, with a Pm structure, is stable at 70 GPa. Our electronic band structure calculations indicate that the substitution of Mg increases the bandgaps of the Mg0.25Ca0.75F2 compounds above 70 GPa.