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We recently proposed a high-pressure and high-temperature P-62m-symmetrypolymorph for CaF2 on the basis of ab-initio random structure searching anddensity-functional theory calculations [Phys. Rev. B 95, 054118 (2017)]. Werevisit this polymorph using both ab-initio and classical molecular dynamicssimulations. The structure undergoes a phase transition to a superionic phasein which calcium ions lie on a bcc-symmetry lattice (space group Im-3m), aphase not previously discussed for the group-II difluorides. We demonstratethat modelling this phase transition is surprisingly difficult, and requiresvery large simulation cells (at least 864 atoms) in order to observe correctqualitative and quantitative behaviour. The prediction of superionic behaviourin P-62m-CaF2 was originally made through the observation of a latticeinstability at the harmonic level in DFT calculations. Using superionicalpha-CaF2, CeO2, beta-PbF2 and Li2O as examples, we examine the potential ofusing phonons as a means to search for superionic materials, and propose thatthis offers an affordable way to do so.

High-pressure CaF2 revisited: a new high-temperature phase and the role of phonons in the search for superionic conductivity