Defect chemistry of the mixed conducting cage compound Ca 12Al14O33

Abstract

The electrical transport properties of mayenite (Ca12Al 14O33 or 12CaO·7Al2O3; mostly abbreviated as C12A7) can be controlled in a wide range by varying the oxygen deficiency: At high temperatures mayenite becomes either an oxygen solid electrolyte, a mixed ionic/electronic conductor or an inorganic electride with metal-like properties upon chemical reduction (removing oxygen). The underlying defect chemistry can be understood on the basis of a relatively simple model - despite the complex cage structure: A point defect model based on the assumption that the framework [Ca12Al14O 32]2+ acts as a pseudo-donor describes well the high temperature transport properties. It accounts for the observed conductivity plateau at higher oxygen activities and also describes the experimentally observed oxygen activity dependence of the electronic conductivity with -1/4 slope at temperatures between 800 and 1000°C. Doping effects in mayenite are still not well explored, and we review briefly the existing data on doping by different elements. Hydration of mayenite plays a crucial role, as Mayenite is hygroscopic, which may be a major obstacle for technical applications.