Fig. 1. Crystal structures of MgF₂ with (a) the rutile-type phase and (b) the fluorite-type phase. The large and small spheres represent magnesium and fluorine atoms, respectively.

Fig. 2. Calculated enthalpy as a function of pressure in the framework of (a) GGA and (b) LDA for MgF₂ with the rutile-type and fluorite-type structures at zero temperature. In the inset, the relative enthalpy versus pressure is presented.

Fig. 3. Calculated band structures of MgF₂ using GGA method: (a) The rutile-type phases at 0 and 19.26 GPa; (b) the fluorite-type phase at 19.26 and 135 GPa.

Fig. 4. Calculated phonon spectra of MgF₂ with the rutile-type phases at (a) 0 GPa and (b) 19.26 GPa and with (c) the fluorite-type phase at 135 GPa using GGA method.

Fig. 5. Pressure dependence of the elastic constants C_ij for MgF₂ with the fluorite-type structure.

Fig. 6. Volume ratios of MgF₂ with the fluorite-type structure obtained from molecular dynamics simulations and first-principles calculations: (a) Volume ratios under different pressures at 300 K; (b) volume ratios under different temperatures at 50 GPa, where in the inset, the data at 0.1 MPa is presented.

Fig. 7. Energy as a function of primitive cell volume for MgF₂ with the fluorite-type structure using GGA and LDA calculations.

Fig. 8. (a) Molar volume and (b) total energy of MgF₂ with the fluorite-type structure as a function of temperature under different pressures calculated by molecular dynamics.

Fig. 9. Melting phase diagram of MgF₂ with the fluorite-type structure obtained from molecular dynamics (MD) simulations, in comparison with the calculated melting phase diagram of the rocksalt phase of MgO.

Fig. 10. Predicted volume thermal expansion coefficient α, isothermal bulk modulus K_T, and thermoelastic parameter αK_T of MgF₂ with the fluorite-type structure as a function of pressure at 300 K and other different temperatures (500, 1000 and 1500 K).

Fig. 11. Predicted volume thermal expansion coefficient α, isothermal bulk modulus K_T, and thermoelastic parameter αK_T of MgF₂ with the fluorite-type structure as a function of temperature at 0.1 MPa and other different pressures (50, 100 and 135 GPa).