Abstract: Detailed density functional theory (DFT) calculations of the structural, mechanical, thermodynamic, and electronic properties of crystalline CaF₂ with five different structures in the pressure range of 0 GPa-150 GPa are performed by both GGA (generalized gradient approximation)-PBE (Perdew-Burke-Ernzerhof) and LDA (local density approximation)-CAPZ (Cambridge Serial Total Energy Package). It is found that the enthalpy differences imply that the fluorite phase $ \to $ PbCl₂-type phase $ \to $ Ni₂In-type phase transition in CaF₂ occurs at P _GGA1 = 8.0 GPa, P _GGA2 = 111.4 GPa by using the XC of GGA, and P _LDA1 = 4.5 GPa, P _LDA2 = 101.7 GPa by LDA, respectively, which is consistent with previous experiments and theoretical conclusions. Moreover, the enthalpy differences between PbCl₂-type and Ni₂In-type phases in one molecular formula become very small at the pressure of about 100 GPa, indicating the possibility of coexistence of two-phase at high pressures. This may be the reason why the transition pressure of the second phase transition in other reports is so huge (68 GPa-278 GPa). The volume changed in the second phase transition are also consistent with the enthalpy difference result. Besides, the pressure dependence of mechanical and thermodynamic properties of CaF₂ is studied. It is found that the high-pressure phase of Ni₂In-type structure has better stiffness in CaF₂ crystal, and the hardness of the material has hardly changed in the second phase transition. Finally, the electronic structure of CaF₂ is also analyzed with the change of pressure. By analyzing the band gap and density of states, the large band gap indicates the CaF₂ crystal is always an insulator at 0 GPa-150 GPa.

Key words: density functional theory (DFT), high-pressure, phase transition, coexistence