Effects of Mg substitution on electronic properties of CaF2

Abstract

We investigated the phase transitions of CaF2, using the state-of-the-art density functional theory. The calculations show that the fluorite structure, with the space group , transforms into the cotunnite structure, with the space group Pnma, at 8 GPa. The electronic band structure of the Pnma-CaF2 structure can be classified as an insulating phase with a wide bandgap, and the bandgap decreases under pressure above 70 GPa. Also, we proposed the 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 compound with a Pm structure is stable at 70 GPa. The results of the electronic band structure reveal that the substitution of Mg enhances the bandgaps of the Mg0.25Ca0.75F2 compounds under pressure above 70 GPa, compared with those of CaF2.