A Radioactive Tracer Study of Diffusion Processes in Lead and Silver Bromide

Anion self-diffusion has been studied in PbBrz and AgBr using an isotope exchange technique. In addition the diffusion of Pb in PbBr? has been measured using 2loPb as tracer and a chemical sectioning method. The effect of bromine gas pressure on anion diffusion has been examined for both materials. In the case of PbBr. these observations have been supplemented by ionic conductivity measurements. For PbBrl the results indicate that the dominant defects are Schottky vacancies with a formation energy of 1.65 eV and an anion mobility energy of 0.31 eV. Interpretation of the results for AgBr is more difficult since the Arrhenius plot of the diffusion coefficient over the range 10-17 < D < 10-9 (cm?s 1 ) shows no clear-cut linear region. The pressure variation of the diffusion coefficient, however, strongly suggests that a single anion vacancy mechanism does not operate to any extent. The kinetics of bromine release during photo-deconiposition in UV light has been studied for AgBr and PbBrz. In both cases the kinetics indicate that a diffusion controlled step is rate limiting a t long times.