Reactions among Point Defects in Alkali Halides

Abstract

Existing knowledge of point defects and their interactions in uncolored alkali halide crystals is briefly reviewed. Attention is drawn to recent experimental and theoretical work which indicates that vacancy pairs are not highly mobile defects. This has important implications for some hypotheses on the kinetics of color center reactions at low temperatures. This knowledge of defects in uncolored crystals is then applied to a discussion of the reactions which may take place between F centers and impurity ions in additively-colored doped crystals and the theoretical possibilities are compared with experimental information on Z centers. Consideration of the thermodynamic equilibria in these reactions seems to eliminate the Z1 and Z2 centers of Pick as possible causes of either of the Z1 and Z2 absorption bands in KCl. (The Z2 model of Pick is identical with the Z1 model of Seitz and is a divalent impurity cation which has trapped an electron.) Two possible centers for the Z2 band presen tthemselves; (a) an associate of an F center with a divalent cation and (b) an associate of an F center with an impurity-vacancy pair (the Z2 model of Seitz). The structure of the Z1 center is not resolved by these considerations. Attention is drawn to some paradoxical experimental results on Z1 and Z2 centers and further experiments to resolve outstanding questions are suggested.