In-situ x-ray diffraction of a shock-induced phase transition in fluorite, CaF$_{\mathrm{2}}$

The difluorides are an important class of ionic compounds that show extensive polymorphism under both static and dynamic loading. In this study, the shock-induced phase transitions in CaF$_{\mathrm{2\thinspace }}$were investigated by in situ x-ray diffraction measurements in plate impact experiments carried out with the two-stage gas gun at the Dynamic Compression Sector of Argonne National Laboratory. Single-crystal samples in (100) and (111) orientations were shock loaded to pressures between 32 GPa to 70 GPa. The particle velocities at the interface between the sample and a LiF window were measured by VISAR and PDV. Synchrotron x-ray diffraction data were recorded at 153.4 ns intervals using a four-frame detector. The measured diffraction patterns show a high degree of sample texturing at all pressures. We observe evidence for a transition to a high-pressure phase followed by reverse transformation at late times during release. This study provides the first direct constraints on the high-pressure lattice structure of fluorite under shock compression.