Design Rules for High Oxygen-Ion Conductivity in Garnet-Type Oxides

We performed a first-principles screening on 90 types of garnet-type oxides (A3B2C3O12) to consider their potential as oxygen ion conductors based on prior knowledge of Ca3Fe2Ge3O12. We identified several oxides that are energetically and dynamically stable and have a low migration barrier energy (Emig) for interstitial oxygen (Oi) on the delocalized migration path. Among them, Cd3Sc2Ge3O12, Ca3Y2Ge3O12, Ca3In2Ge3O12, and Ca3Ga2Ge3O12 may be able to generate Oi by donor dopants such as LaCa+ and InCd+. On the basis of the calculations by first-principles molecular dynamics, we expect that these oxides may have oxygen ion conductivities (σO) of a similar order of magnitude to that of the reference level of ∼10–2 S/cm exhibited by yttria-stabilized zirconia at 1000 K. We provide scientific reasoning and design rules regarding the high σO of garnet-type oxides. For example, we find that the low Emig of Oi is attributed to smaller deviations in the atomic radii of cations and bond lengths.