Electrochemical Cells and Electrical Conduction of Pure and Doped Al 2 O 3

Data concerning the types of charge carrier responsible for electrical conduction in single‐crystal aluminum oxide are reviewed, and explanations are offered for discrepancies in terms of the experimental conditions. Measurements on undoped and Co 2+ and Mg 2+ doped crystals, made using a volume guard to avoid surface and gas conduction, are used to describe Al 2 O 3 as an ionic conductor with Al interstitial ions as the principal charge carriers at high oxygen activity. A defect model is proposed and the mobility of Al i 3+ , the concentration of Al i 3+ in undoped crystals, and the equilibrium constant for defect formation at high temperatures are estimated.