Defects and defect processes in ionic oxides: Where do we stand today?

The many faceted problem of defects in particle-bombarded ionic oxides is bewildering in its complexity yet fascinating in its appeal. In the past dozen years progress has been made in sorting out numerous problems of defect structure and ctronic behavior. In this survey our present situation will be assessed and pressing problems blocking progress will be indicated. Identification of simple point defects in a number of oxides including the alkaline earths, sapphire and spinel, has proceeded to the point where there is now a well defined set of tools for assaying the nature of lattice damage and transport mechanisms in these and related oxides. The electronic behavior of these simple defects (anion and cation vacancies and aliovalent impurities in various charge states) pose interesting problems in their own right: Optical transformation of F ⁺ to F centers in α-Al ₂O₃ is a puzzle which has stimulated a new theory of the electronic structure of the O ^2- vacancy which promises to have an impact on studies of electron trapping defects in other structures. Although the situation at low damage levels is reassuring, the bridge from this beachhead to the heavy damage regime in which the complexity of damage is immense has yet to be built. Strategies for bridging this chasm will be suggested.