Experimental evidence of the mixed-coordinated rhombohedral phase in magnesium fluoride under high pressure

Magnesium fluoride (MgF₂) serves as an important analogue system to study the pressure-induced structural and electronic phase transitions in oxides relevant to geoscience and planetary science, such as SiO₂ and GeO₂. In this work, through first-principles calculations combined with synchrotron X-ray diffraction and Raman-scattering measurements in laser-heated diamond anvil cells, we studied the structural phase transition of MgF₂ at high pressure. We confirmed the existence of a mixed coordinated rhombohedral phase with R3ˉR3̄ R3ˉ symmetry in MgF₂, which was previously predicted to be thermodynamically stable between 645 and 890 GPa in silica, but has never been synthesized experimentally. This R3ˉR3̄ R3ˉ phase is the ground state in the pressure range from 40 GPa to 55 GPa in MgF₂, in analogy to silica but at much lower pressure. The in-situ high pressure XRD measurements confirm the phase transition from the pyrite-type phase to the R3ˉR3̄ R3ˉ phase after laser heating at around 51 GPa along with the new Raman modes well aligned with the calculated R3ˉR3̄ R3ˉ phase. Furthermore, our extended calculations on GeO₂ confirms the R3ˉR3̄ R3ˉ phase as the ground state at 250–310 GPa. Considering the similar phase transition sequences of silica and magnesium fluoride, our findings provide a reference for the future experimental verification of R3ˉR3̄ R3ˉ phase SiO₂, which is of great significance in geoscience and planetary science.