It was recalled that defect-free crystals did not conduct ions, and that increased disorder initially enhanced atomic transport. However, heavily defective crystals often exhibited successively lower levels of atomic diffusivity as the defect level was increased. This anomaly could be resolved by noting that highly disordered solids tended towards order. Interesting insights could be obtained by considering compounds with related structures which could be induced to go through order-disorder transitions by changing the temperature or the composition. A number of such systems were considered, including materials with structures which were related to fluorites and perovskites.

H.L.Tuller: Journal of the Physics and Chemistry of Solids, 1994, 55[12], 1393-404