High Pressure-Temperature Study of MgF, CaF, and BaF by Raman Spectroscopy: Phase Transitions and Vibrational Properties of AF Difluorides.

The phase transition of AF difluorides strongly depends on pressure, temperature, and cationic radius. Here, we have investigated the phase transition of three difluorides, including MgF, CaF, and BaF, at simultaneously high pressures and temperatures using Raman spectroscopy and X-ray diffraction in externally heated diamond anvil cells up to 55 GPa at 300-700 K. Rutile-type difluoride MgF with a small cationic radius undergoes a transition to the CaCl-type phase at 9.9(1) GPa and 300 K, to the HP-PdF-type phase at 21.0(2) GPa, and to the cotunnite-type phase at 44.2(2) GPa. The phase transition pressure to the HP-PdF and cotunnite structure at 300 K for our single crystal was found to be higher than that in previous studies using polycrystalline samples. Elevating the temperature increases the transition pressure from rutile- to the CaCl-type phase but has a negative influence on the transition pressure when MgF transforms from the HP-PdF- to cotunnite-type phase. Meanwhile, the transition pressure from the CaCl- to HP-PdF-type phase for MgF was identified to be independent of the temperature. Raman peaks suspected to belong to the α-PbO-type phase were observed at 14.6-21.0(1) GPa and 400-700 K. At 300 K, difluorides CaF and BaF in the fluorite structure with larger cationic radii transform to the cotunnite-type phase at 9.6(3) and 3.0(3) GPa at 300 K, respectively, and BaF further undergoes a transition to the NiIn-type phase at 15.5(4) GPa. For both CaF and BaF, elevating the temperature leads to a lower transition pressure from fluorite- to the cotunnite-type phase but has little influence on the transition to the NiIn structure. Raman data provide valuable insights for mode Grüneisen parameters. We note that the mode Grüneisen parameters for both difluorides and dioxides vary linearly with the cation radius. Further calculations for the mode Grüneisen parameters at high pressures for MgF, CaF, and BaF yield a deeper understanding of the thermodynamic properties of the difluorides.