High-Pressure Phase Diagram and Superionicity of Alkaline Earth Metal Difluorides

We study the high-pressure–high-temperature phase diagram and superionicity of alkaline earth metal (AEM) difluorides (AF₂, A = Ca, Sr, Ba) with first-principles simulation methods. We find that the superionic behavior of SrF₂ and BaF₂ at high pressures differ appreciably from that previously reported for CaF₂ [Phys. Rev. Lett. 2014, 113, 235902]. Specifically, the critical superionic temperature of SrF₂ and BaF₂ in the low-pressure cubic fluorite phase is not reduced by effect of compression, and the corresponding high-pressure orthorhombic contunnite phases become superionic at elevated temperatures. We get valuable microscopic insights into the superionic features of AEM difluorides in both the cubic fluorite and orthorhombic contunnite phases by means of ab initio molecular dynamics simulations. We rationalize our findings on the structural and superionic behavior of AF₂ compounds in terms of simple ionic radii arguments and generalize them across the whole series of AEM dihalides (AB₂, A = Mg, Ca, Sr, Ba and B = F, Cl, Br, I) under pressure.