Dynamic compression of [100] MgF2 single crystals: Shock-induced polymorphism to highly coordinated structures

Abstract

${AX}_{2}$-type compounds exhibit a rich and complex series of phase transitions under compression. The high-pressure polymorphism of rutile-type ${AX}_{2}$ difluorides can be characterized by a typical sequence from $\mathrm{rutile}\phantom{\rule{0.28em}{0ex}}\mathrm{type}\ensuremath{\rightarrow}\mathrm{Ca}{\mathrm{Cl}}_{2}\phantom{\rule{0.28em}{0ex}}\mathrm{type}\ensuremath{\rightarrow}\mathrm{HP}\text{\ensuremath{-}}\mathrm{Pd}{\mathrm{F}}_{2}\phantom{\rule{0.28em}{0ex}}\mathrm{type}\ensuremath{\rightarrow}\mathrm{cotunnite}\phantom{\rule{0.28em}{0ex}}\mathrm{type}$. Relative to dioxides, the reduced valence and ionic radius of the ${\mathrm{F}}^{\ensuremath{-}}$ anion in rutile-type ${\mathrm{MgF}}_{2}$ (sellaite) results in lowered transition pressures, making it a useful analog for ${\mathrm{SiO}}_{2}$ and other dioxides. In this work, ${\mathrm{MgF}}_{2}$ single crystals were shock-compressed along the [100] direction to 24 to 120 GPa using plate impact techniques, and wave profiles were measured using laser interferometry. At low stresses (24 to 44 GPa), we observe features consistent with an elastic--inelastic response, followed by a phase transformation. Peak stress-density states in this stress range are consistent with those expected for the modified fluorite-type (HP-${\mathrm{PdF}}_{2}$) phase or a related structure. At higher stresses (69 to 91 GPa), we observe a two-wave structure with peak stress-density states consistent with transformation to a denser and likely more highly coordinated phase such as 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.