Superionicity and Polymorphism in Calcium Fluoride at High Pressure

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.

5 pages, 5 figures

References in corpus (2)

• Temperature of the inner-core boundary of the Earth: Melting of iron at high pressure from first-principles coexistence simulations
• Ab initio melting curve of molybdenum by the phase coexistence method

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