Anion self-diffusion in AgCl and AgBr by both single vacancies and vacancy pairs

Both isolated anion vacancies and vacancy pairs are found to contribute to the anion mass transport in AgCl and AgBr, as determined from the anion tracer self-diffusion in pure and in heavily doped specimens. The data were analysed by means of the Lidiard-Debye-Huckel theory, despite some very serious questions as to the validity of this model when the defect concentrations are as large as in the present experiments. In the case of AgCl, the diffusion parameters appear to be normal, and both pairs and single vacancies are found to make comparable contributions to anion diffusion. In AgBr, however, the effective activation energies and entropies appear to be very anomalously large, especially those for the single vacancy, indicating the involvement of some phenomenon not included in the usual simple model of point defects in ionic crystals.