Electronic and elastic properties of CaF2 under high pressure from ab initio calculations

Calcium fluoride CaF2 has been studied by using density functional theory (DFT) with the generalized gradient approximation (GGA). Our results demonstrate that the sequence of the pressure-induced structural transition of CaF2 is the fluorite structure (Fm (3) over barm), the orthorhombic cotunnite-type structure (Pnma), and the hexagonal Ni2In-type structure (P6(3)/mmc). The two transitions occur at pressures of 8 GPa and 105 GPa, accompanied by volume collapses of 8.4% and 1.2%, respectively. The energy band gap increases with pressure in the (Fm (3) over barm) and the forepart of Pnma phases. However, on increasing the pressure beyond 60 GPa, the gap decreases, which is due to the fluorine p(z)-states shifting toward the Fermi energy. In addition, the elastic properties versus pressure are also discussed. Our calculated elastic constants for the cubic phase at ambient pressure are in agreement with the experimental values. The stress-strain coefficient calculations show that shear transformations in the Pnma phase are more difficult than in the cubic phase and the compressibility along the c(h) (or a(o)) direction for the orthorhombic phase is stronger than that in the hexagonal crystal.