Shock response of [100] MgF₂ single crystals to 120 GPa

Under static compression, the high-pressure polymorphism of rutile-type difluorides can be characterized by a common sequence from rutile CaCl₂ HP-PdF₂ cotunnite. Relative to dioxides, the reduced valence and ionic radius of the F^- anion in rutile-type MgF₂ (sellaite) results in decreased transition pressures, making it an ideal analogue for stishovite (SiO₂) which has been predicted to undergo the HP-PdF₂ to cotunnite-type transformation at pressures in excess of 600 GPa. In plate impact experiments conducted at the Institute for Shock Physics, laser interferometry (VISAR and PDV) was used to measure wave profiles for [100]-oriented MgF₂ single crystals shock compressed to 24 – 120 GPa. At low stresses (24-44 GPa), we observe wave profile features consistent with elastic-inelastic response, followed by a phase transformation. Peak stress-density states in this stress range are consistent with the modified fluorite-type (HP-PdF₂) structure. At higher stresses (69-91 GPa), we observe a two-wave structure with peak stress-density states showing good agreement with the cotunnite-type structure. At 120 GPa, only a single wave structure is observed, indicating that the wave profile features observed at lower stresses are overdriven.