Mechanical and optical response of magnesium fluoride single crystals shock compressed along the c-axis

Magnesium fluoride (MgF₂), which occurs naturally as the mineral sellaite, is of significant interest to geophysics because its ambient rutile-type structure is isomorphic to SiO₂ (stishovite). Magneisum fluoride is also used extensively as an optical material. In this work, plate impact experiments were carried out to study the mechanical and optical response of single crystal MgF₂ samples shock compressed to 130 GPa along the c-axis. Resulting wave profiles and shock velocities through the samples were measured using laser velocimetry (PDV). Shocked MgF₂ exhibits a nonlinear elastic response up to ~10 GPa, beyond which a two-wave structure was observed. A single overdriven wave was recorded for peak longitudinal stresses above 95 GPa. In the absence of x-ray diffraction data, it was not possible to determine whether the observed two-wave structure resulted from inelastic deformation or due to phase transitions reported under static high-pressure loading (rutile → orthorhombic → distorted fluorite → cotunnite). Our experiments also show that c-axis MgF₂ remains optically transparent to 1550 nm wavelength light for the entire stress range examined here and therefore, it can be used as an optical window in shock wave experiments. LA-UR-22-22137