Diffusion of cobalt ion in single crystals of silver bromide

Abstract

The diffusion coefficients of divalent cobalt ion have been measured in single crystals of highpurity silver bromide in the temperature range 243–408°C, using the tracer-sectioning method. Their values ranged from about 2 × 10 −10 cm 2 s −1 to 2 × 10 −7 cm 2 s −1. The diffusion Arrhenius plot is found to be comprised of two linear segments with activation energies of 1.15 eV and 1.39eV for the regions below and above 338°C, respectively. The data are interpreted in terms of diffusion by means of impurity-vacancy complexes. The diffusion coefficient of the complex as a function of temperature was determined from the measured tracer diffusivity and the degree of impurity-vacancy association. These data, with the exception of three points at the highest temperatures, followed an Arrhenius relation from which the enthalpy and entropy for the jump of Co 2+ into a nearest-neighbor cation vacancy were deduced to be 0.74 eV and 3.9 k B , respectively. An alternative analysis, though naive, in which the tracer diffusivity was normalized by the cation vacancy concentration surprisingly removed the curvature from the diffusion Arrhenius plot. In contrast to the case for AgCl, the results of the present study and some others reported in the literature show no systematic variation in the migration energy with the electronic structure of the solute ion.