Point defects in the silver halides—what one learns from ion transport and surface potential

Abstract

This paper reviews the understanding that has been gained about point defects and ion transport in AgCl and AgBr. One now has considerable information about the mechanisms of cation interstitial migration, including the temperature dependence of the Frenkel defect formation energy and also the extraordinarily low values of the migration activation energy. It has been possible to resolve the formation enthalpy and entropy of the defect pair into the separate contributions from the cation vacancy and the interstitial. One conclusion from this analysis is that both the relatively low formation free energy of the Frenkel pair and the negative surface potential result from a very large relaxation of phonon frequencies upon insertion of an interstitial silver ion. The contrasting dynamic behaviours of independent interstitials (as in AgBr) and interacting interstitials (as in AgI) are illustrated by measurements of the ionic Hall effect. Finally, the diffusion of halide ions (both self-diffusion and the diffusion of impurity halides) is discussed, with emphasis on the contributions from single vacancies and vacancy pairs; these measurements reveal an anomalously large pre-exponential factor which has thus far not been explained.