We present a combined experimental and computational first-principles study of the superionic and structural properties of CaF2 at high P-T conditions. We observe an anomalous superionic behavior in the low-P fluorite phase that consists in 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 which 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.
@article{arxiv.1405.5992,
title = {Superionicity and Polymorphism in Calcium Fluoride at High Pressure},
author = {Claudio Cazorla and Daniel Errandonea},
journal= {arXiv preprint arXiv:1405.5992},
year = {2015}
}