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

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 The prediction of superionic behaviour in P-62m-CaF2 was originally made through the observation of a lattice instability at the harmonic level in DFT calculations.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