First-principles study of structural stabilities, and electronic and optical properties of CaF2 under high pressure

An investigation into the structural stabilities and the electronic and optical properties of CaF2 under high pressure was conducted using first-principles calculations based on density functional theory. Our results demonstrate that the sequence of the pressure-induced phase transition of CaF2 is the fluorite structure (Fm3m), the PbCl2-type structure (Pnma), and the Ni2In-type structure (P6(3)/mmc). At these phase transformations, the coordination number of Ca2+ increases from eight to nine and then to eleven. The mechanisms of the structure change were revealed from the PbCl2-type phase to the Ni2In-type phase. The energy band gap increases with pressure in the Fm3m and the Pnma phases, but decreases in the P6(3)/mmc phase. The band gap pressure coefficients were obtained using a linear pressure-dependent fit function. In addition, the energy band overlap metallization does not occur up to 218 GPa. The static dielectric constants epsilon(0) vs pressure are also discussed.