Determination of ionic transport processes in AgCl and AgBr

Abstract

AgCl and AgBr have many unusual defect properties. Therefore a review is given of the experimental techniques and conceptual models that have been applied to these materials. A brief discussion is presented of the interpretation of deviations from the normal Einstein relation between conductivity and diffusion on the basis of correlation and displacement effects. Examples are given of the use of doped conductivity, tracer diffusion, and isotope effect experiments for the elucidation of cation vacancy and collinear and noncollinear interstitialcy jump processes. A detailed fitting of the temperature dependence of the ionic conductivity in an intermediate temperature range, with inclusion of complex association and Debye-Hückel corrections, provides formation and migration enthalpies and entropies. A large conductivity anomaly at higher temperatures requires the introduction of a temperature dependent correction of the formation energy. Comparison to Na diffusion in these materials indicates that most of the anomaly must be due to enhanced defect concentrations rather than lowered migration energies. The Debye-Hückel corrections can account for only a fraction of the total effect, and a rough calculation shows that the unusual rise in the dielectric constant can give at best a qualitative explanation. A better theoretical calculation of the general influence of softening of the lattice on defect energies is called for