Oxygen ion conduction in 12CaO·7Al₂O₃: O^2- conduction mechanism and possibility of O^- fast conduction

12CaO·7Al₂O₃ (C12A7) with a unique nano-porous structure and free O^2- ions entrapped in sub-nanometer-sized cages is a fast oxygen-ion-conducting material. These free O^2- may be replaced by various oxygen-related species, OH^-, O^2- and O^-, by tuning the atmosphere during the heat treatment. We examined the conduction mechanism for stoichiometric C12A7 (C12A7:O^2-), in which O^2- ions exist as counter anions in sub-nanometer-sized cages, by Raman measurement of C12A7:O^2- annealed in a dry ¹⁸O₂ atmosphere. It was revealed that the primary ion conducting species is an O^2- ion which diffuses via exchange with O^2- in the cage wall. An experimental result on the sample containing O^- ions implied that O^- is more mobile than O^2- in C12A7. Ab initio calculations on the diffusion paths of O^2- and O^- ions in C12A7 supported the above experimental results.