Electronic and elastic properties of CaF₂ under high pressure from abinitio 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 (), the orthorhombic cotunnite-type structure (Pnma), and the hexagonal Ni₂In-type structure (P6₃/mmc). The two transitions occur at pressures of 8GPa and 105GPa, accompanied by volume collapses of 8.4% and 1.2%, respectively. The energy band gap increases with pressure in the and the forepart of Pnma phases. However, on increasing the pressure beyond 60GPa, 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.