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 (Oi) with the kick-out mechanism than that (0.76 eV) for an oxygen vacancy. The migration paths for Oi are delocalized and connected to the neighboring cells in three-dimensional space. This oxide does not have a very low formation energy of Oi 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 Oi provides a good oxygen-ion conductivity, along with the electronic conductivity.