It was recalled that many metal oxides, with fluorite- and perovskite-related structures, were oxide-ion conductors. Structural and thermodynamic parameters, such as the critical radius of the pathway for oxide-ion movement, the free-lattice volume and the average metal–oxide bond energy had been proposed as predictors of high oxide-ion conductivity. It was considered here how these parameters all depended upon the ionic radii and, therefore, some of them might be redundant. The interrelationships among such parameters, for fluorite and perovskite oxides, were explored by considering their sensitivities to individual ionic radii. Based upon published experimental data, it was concluded that lattice distortion (lattice stress and deviation from cubic symmetry) - due to ion-radius mismatch – very largely determined the ionic conductivity. The lattice distortion was of much greater importance than were many other proposed parameters. In the case of perovskites, the charge on the B-site ion was also of major importance.

Factors Controlling the Oxide Ion Conductivity of Fluorite and Perovskite Structured Oxides. M.Mogensen, D.Lybye, N.Bonanos, P.V.Hendriksen, F.W.Poulsen: Solid State Ionics, 2004, 174[1-4], 279-86