Diffusion of Cadmium in Pure and Cadmium Doped AgBr

Abstract

The diffusion of Cd in pure AgBr and dilute AgBr‐CdBr₂ solid solutions was measured by a tracer technique using carrier‐free Cd¹⁰⁹. It was necessary to use the extremely high specific activity carrier‐free isotope because the diffusion of Cd in AgBr depends sensitively on impurity concentration. The data support Lidiard's theory for the diffusion of divalent impurities in monovalent ionic crystals. From the diffusion rates in the pure material, it is possible to calculate the association energy ΔG of Cd⁺⁺Ag vacancy complexes. The value of ΔG extrapolated to 0°K is estimated to be 0.21±0.04 ev. The observed concentration dependence of Cd diffusion in doped samples can be explained only if the mobility of complexes is assumed to have a strong concentration dependence. This interpretation suggests that a reappraisal of the analysis of previous electrical conductivity and thermoelectric power doping experiments, in which the mobility was assumed to be concentration independent, would be desirable.