Oxygen ion conduction in 12CaO·7Al2O3: O2− conduction mechanism and possibility of O− fast conduction

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