COMMENTS ON DIFFUSION BY DIVALENT IONS IN ALKALI AND SILVER HALIDES

The theory of diffusion by complexes of divalent cations and cation vacancies is reviewed. First the microscopic diffusion coefficients for both the divalent ion and the host ion are expressed in terms of appropriate jump frequencies and correlation factors. Next the concentration dependence of the macroscopic diffusion coefficient for divalent ions is expressed in terms of the degree of association of the complexes and their distribution in the sample. A number of experimental arrangements are considered, involving chemical diffusion and tracer diffusion with and without carrier, and a number of examples are given of both correct and incorrect application of the analysis for experiments in the alkali halides. The results for diffusion of Cd in AgCl are re-examined, and it is suggested that inclusion of the concentration dependence indicates that Cd diffusion occurs by means of complexes at all temperatures. For Cd in AgBr it appears that proper treatment of the concentration dependence introduced by the complexes may remove the need for most or all of the previously proposed increase of mobility with concentration. Finally the relative contribution of complexes to diffusion of Ag and Cd in AgCl : Cd is obtained for use in interpreting experiments on tracer diffusion ofAg in this system. © 1969.