Superionicity and Polymorphism in Calcium Fluoride at High Pressure

TL;DR

This study combines experiments and first-principles calculations to explore superionic and structural behaviors of CaF2 under high pressure and temperature, revealing unexpected phase-dependent superionicity and a new high-T monoclinic phase.

Contribution

It uncovers anomalous superionic behavior in CaF2's fluorite phase and identifies a novel high-temperature monoclinic phase with superionicity, advancing understanding of high-pressure ionic conductors.

Findings

Superionic transition temperature decreases with pressure in fluorite phase.

Superionicity is absent in high-pressure cotunnite phase.

A new monoclinic high-T phase exhibits superionic conductivity.

Abstract

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.