Superionicity and polymorphism in calcium fluoride at high pressure

We present a combined experimental and computational first-principles study of the superionic and structural properties of CaF_{2} at high P-T conditions. We observe an anomalous superionic behavior in the low-P fluorite phase that consists of a decrease of the normal → superionic critical temperature with compression. This unexpected effect can be explained in terms of a P-induced softening of a zone-boundary X phonon that involves exclusively fluorine displacements. Also we find that superionic conductivity is absent in the high-P cotunnite phase. Instead, superionicity develops in a new low-symmetry high-T phase that we identify as monoclinic (space group P2_{1}/c). We discuss the possibility of observing these intriguing phenomena in related isomorphic materials.