Electronic and elastic properties ofCaF2 under high pressure from ab initio calculations

Abstract

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 ofCaF2 is the fluorite structure (), the orthorhombic cotunnite-type structure (Pnma), and thehexagonal Ni2In-typestructure (P63/mmc). The two transitions occur at pressures of 8 GPa and 105 GPa, accompanied by volumecollapses of 8.4% and 1.2%, respectively. The energy band gap increases with pressure inthe and the forepart of Pnma phases. However, on increasing the pressure beyond 60 GPa, the gap decreases, which is due to thefluorine pz-states shifting toward the Fermi energy. In addition, the elastic properties versuspressure are also discussed. Our calculated elastic constants for the cubicphase at ambient pressure are in agreement with the experimental values. Thestress–strain coefficient calculations show that shear transformations in thePnma phase are more difficult than in the cubic phase and the compressibility along thech (orao) direction for the orthorhombic phase is stronger than that in the hexagonal crystal.