Shock-induced polymorphic transitions of Pb F 2 up to 1 TPa and their implications for the universal behavior of shocked A X 2 compounds

Hugoniot data for $\\mathrm{Pb}{\\mathrm{F}}_{2}$ single crystals were measured for pressures up to 1 TPa through gas gun and laser shock experiments. Experimental results show that $\\mathrm{Pb}{\\mathrm{F}}_{2}$ transforms to a less compressible phase at 29 GPa and possibly melts at 57 GPa. Quantum molecular-dynamics simulations were also performed to calculated the liquid Hugoniot for $\\mathrm{Pb}{\\mathrm{F}}_{2}$. The simulated results are in good agreement with the experimental results, supporting the high-pressure phase is liquid. The current study, combined with previous studies on $\\mathrm{Ti}{\\mathrm{O}}_{2}, \\mathrm{Si}{\\mathrm{O}}_{2}$, and $\\mathrm{Ca}{\\mathrm{F}}_{2}$, suggests that for $A{X}_{2}$ compounds, decreasing of Hugoniot slope can be taken as an indication of melting, and the Hugoniot slope approaches 1.2 at ultrahigh pressure.