Phase transitions of AF2 difluorides MgF2, CaF2, and BaF2 at high pressures and temperatures

SiO₂ is a crucial oxide in Earth and other terrestrial planets such as super-Earth, making the study of its high-pressure structural phase transitions and physical properties vital. Due to the extremely high pressures required for the phase transitions of SiO₂, significant uncertainty exist in the high-pressure phase boundaries from experiments and theoretical calculations. Given the similarities in high-pressure structures between SiO₂ and the AF₂ difluorides, the latter serve as excellent analogs for studying the high-pressure properties of SiO₂. The phase transition of AF₂ difluorides strongly depends on cationic radius, pressure, and temperature. In this study, we investigated the phase transitions of MgF₂, CaF₂, and BaF₂ at high pressures and temperatures using Raman spectroscopy and X-ray diffraction in diamond anvil cells up to 50 GPa at 300-700 K. These difluorides, with cationic radii of 0.72-1.35 Å, reveal the influence of cationic radius on structure of difluorides under extreme conditions.

Our results show that elevating temperature increases the transition pressure from rutile to the CaCl₂-type phase but has a negative influence on the transition pressure when MgF₂ transforms from the HP-PdF₂- to cotunnite-type phase. Meanwhile, the transition pressure from the CaCl₂- to HP-PdF₂-type phase for MgF₂ is identified to be independent of temperature. For both CaF₂ and BaF₂, elevating temperature leads to a lower transition pressure from fluorite to the cotunnite-type phase but has little influence on the transition to Ni₂In structure. Our results are important for exploring the physical properties and the transition sequence of AX₂-type minerals. The information of these difluorides could also help to understand the structure of the Earth and other terrestrial planets.