Solute migration by interstitials and by vacancies: Mn2+and Cd2+in AgCl

Because of their importance in understanding the processes of substitutional and interstitial cation migration, the diffusivities of manganese and of cadmium tracers in silver chloride have been carefully remeasured. The high degree of association between divalent cations and vacancies at high temperatures is described, and the activation energies for jumping of Mn 2+ and of Cd 2+ into a cation vacancy are determined to be 0.72 eV and 0.68 eV, respectively, about 2 1 / 2 times the migration energy of the free unassociated vacancy. Of the many solutes studied thus far in the silver halides, only the monovalent noble metal ions now appear to diffuse in silver halides by means of interstitials. The relative stabilization of the saddle-points of these two mechanisms of diffusion is qualitatively examined in terms of the electronic structure of the solute, and the covalent and the Coulombic interactions.