Magnesium fluoride ($\mathrm{Mg}{\mathrm{F}}_2$) serves as an important analog system to study the pressure-induced structural and electronic phase transitions in oxides relevant to geoscience and planetary science, such as $\mathrm{Si}{\mathrm{O}}_2$ and $\mathrm{Ge}{\mathrm{O}}_2$. In this work, through first-principles calculations combined with synchrotron x-ray diffraction (XRD) and Raman-scattering measurements in laser-heated diamond anvil cells, we study the structural phase transition of $\mathrm{Mg}{\mathrm{F}}_2$ at high pressure. We confirm the existence of a mixed-coordinated rhombohedral phase with $R\overline{3}$ symmetry in $\mathrm{Mg}{\mathrm{F}}_2$, which was previously predicted to be thermodynamically stable between 645 and 890 GPa in silica but has never been synthesized experimentally. This $R\overline{3}$ phase is the ground state in the pressure range from 40 to 55 GPa in $\mathrm{Mg}{\mathrm{F}}_2$, 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 $R\overline{3}$ phase after laser heating at around 51 GPa along with Raman modes well aligned with the calculated $R\overline{3}$ phase. Furthermore, our extended calculations for $\mathrm{Ge}{\mathrm{O}}_2$ confirm the $R\overline{3}$ 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 the $R\overline{3}$ phase of $\mathrm{Si}{\mathrm{O}}_2$, which is of great significance in geoscience and planetary science.

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• Crystal structure
• High-pressure studies
• Structural phase transition
• First-principles calculations
• Pressure techniques
• Raman spectroscopy
• X-ray diffraction