In conventional fluorites, such as CaF 2 , anion transport requires the presence of thermally-generated point defects-vacancies and interstitials-within the anionic substructure . Fluorites are typically anti-Frenkel disordered: some fraction of anions occupy octahedral interstitial sites, leaving an equal number of tetrahedral sites vacant . While additional interstitials and vacancies can be introduced via aliovalent doping, the intrinsic defect concentration depends on the ease with which anti-Frenkel pairs can form, which, in turn, approximately depends on the relative energies of ions occupying the tetrahedral and octahedral anion sites within the fcc cationic host framework. In simple fluorites, the anti-Frenkel-pair formation energy is usually high: for CaF 2 , anion Frenkel-pair formation energies have been calculated as 2.2 eV to 2.9 eV . As a consequence, at low-to-moderate temperatures, these materi-als have low fluoride-ion defect concentrations and corresponding low ionic conductivities .