[S-IT14] Deep Earth Sciences

[SIT14-02] Phase transitions of AF₂ difluorides MgF₂, CaF₂, and BaF₂ at high pressures and temperatures

*Xinyue Zhang¹, Luo Li¹, Yingxin Yu¹, Qingchun Zhang¹, Ningyu Sun¹, Zhu Mao¹, Dongzhou Zhang² (1.University of Science and Technology of China, 2.University of Chicago)

Keywords:Difluorides, Raman spectroscopy, High pressure and temperature, Phase transition

SiO₂ is a crucial oxide component in Earth, as well as in planets within our solar system and super-Earth exoplanets beyond. Therefore, investigating the high-pressure structural phase transitions and physical properties of SiO₂ is of great importance. However, due to the exceedingly high pressure required for the phase transitions of SiO₂, there exists significant uncertainty in the constraints on the high-pressure phase boundaries based on existing experiments and theoretical calculations. Given the striking 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 complex polymorphism and unique physical properties under extreme conditions have captivated wide attention to AF₂ difluorides. The phase transition of AF₂ difluorides strongly depends on cationic radius, pressure, and temperature. In this study, we have investigated the phase transition of three difluorides, including MgF₂, CaF₂ and BaF₂ at simultaneously high pressures and temperatures using Raman spectroscopy and X-ray diffraction in externally-heated diamond anvil cells up to 50 GPa at 300-700 K. These representative difluorides have cationic radii in the range of 0.72-1.35 Å, which can systematically reveal the influence of cationic radius at high pressure-temperature conditions on the structure of difluorides.
Our results show that for rutile-type difluoride MgF₂ with small cationic radius, it undergoes the transition to the CaCl₂-type phase at 9.9(1) GPa and 300 K, to the HP-PdF₂-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-PdF₂ and cotunnite structure at 300 K for our single-crystal was found to be higher than previous studies using polycrystalline samples. 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 was identified to be independent of temperature. At 300 K, difluorides CaF₂ and BaF₂ in the fluorite structure with larger cationic radius transform to the cotunnite-type phase at 9.6(3) GPa and 3.0(3) GPa at 300 K, respectively, and BaF₂ further undergoes a transition to the Ni₂In-type phase at 15.5(4) GPa. 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. Raman data provide valuable insights for the mode Grüneisen parameters. We note that the mode Grüneisen parameters for both difluorides and dioxides vary linearly with the cationic radius. 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.