09:15 〜 09:30
[SIT14-02] Phase transitions of AF2 difluorides MgF2, CaF2, and BaF2 at high pressures and temperatures
キーワード：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.
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.