Shock Compression Response of Calcium Fluoride (CaF$_{\mathrm{2}})$

Fluorite, a textbook crystal structure named after CaF$_{\mathrm{2}}$, is observed in many materials such as Mg$_{\mathrm{2}}$Si, and CeO$_{\mathrm{2}}$. Specifically, CaF$_{\mathrm{2}}$ is a useful material for studying the fluorite structure because it is readily available as a single crystal. Under static compression, CaF$_{\mathrm{2\thinspace }}$is known to have at least three solid phases: fluorite, cotunnite, and a Ni$_{\mathrm{2}}$In phase. Along the Hugoniot CaF$_{\mathrm{2}}$ undergoes a fluorite to cotunnite phase transition, however, at higher shock pressures it is unknown whether CaF$_{\mathrm{2}}$ undergoes another solid phase transition or melts directly from the cotunnite phase. Historical work by Al'shuler \textit{et al }[1]$. $showed that CaF$_{\mathrm{2}}$ became highly incompressible above 100 GPa. In this work, we conducted planar shock compression experiments on CaF$_{\mathrm{2}}$ using Sandia's Z-machine and a two-stage light gun up from 60 GPa to 900 GPa. Additionally, we conducted decaying shock experiments at the Omega Laser Facility to measure temperature along the Hugoniot. We use density functional theory (DFT) based quantum molecular dynamics (QMD) simulations to provide insight into the CaF$_{\mathrm{2\thinspace }}$state along the Hugoniot. We also compare the experimentally measured temperatures to the DFT calculations. [1] L. V. Al'tshuler \textit{et al.} Sov. Phys. Solid State \textbf{15}, 969, (1973)