Time-resolved measurement of high-pressure phase transition of fluorite under shock loading

In situ time-resolved measurements of shock wave profiles for anisotropic fluorite crystals with two different crystal orientations were carried out up to a pressure of 34 GPa that is above the transition pressure for the fluorite to cotunnite phase. They indicate that the Hugoniot elastic limit varies with the crystal orientation and final pressure and that high-pressure phase transition from fluorite to a cotunnite-type structure occurs at 13 GPa in 10–20 ns for CaF2 [100]-oriented crystals and at 17 GPa in more than 50 ns for CaF2 [111]-oriented crystals, respectively. These results are in disagreement with those from static compression. The phase transition at static pressures has been known to be very sluggish, but the present results indicate a large sensitivity of kinetics to the relationship between crystallographic orientation and shock direction, supporting a martensitic mechanism for the fluorite to cotunnite phase transition that is enhanced by the effect of shock-induced shear. It is also helpful to explain the observation that the in situ emission spectra for shocked Eu-doped fluorite became weak and had no shift above ~15 GPa.