Oxygen conduction mechanism in Ca₃Fe₂Ge₃O₁₂ garnet-type oxide

We investigate the oxygen conduction mechanism in a garnet-type oxide, Ca₃Fe₂Ge₃O₁₂, for the first time in detail by first-principle calculations. The nudged elastic band results confirm that this oxide has a lower migration barrier energy (0.45 eV) for an oxygen interstitial (O_i) with the kick-out mechanism than that (0.76 eV) for an oxygen vacancy. The migration paths for O_i are delocalized and connected to the neighboring cells in three-dimensional space. This oxide does not have a very low formation energy of O_i when the Fermi level is near the lowest unoccupied molecular orbital at a high temperature, which implies the possibility of electron doping by high-valence cations. These theoretical results suggest that the doping of Ca₃Fe₂Ge₃O₁₂ for generation of excess O_i provides a good oxygen-ion conductivity, along with the electronic conductivity.