DIFFUSION OF IMPURITY IONS IN SILVER-HALIDES

The diffusion activation enthalpies H of impurity alkali ions in AgCl and AgBr crystals do not scale with ionic radii, as one might surmise, but they do scale with the ions' orbital-moments ratio q. The quantity q serves as a measure for interaction dominance in the delicate balance between van der Waals attraction and short-range repulsion. In order to explain the observed constant activation enthalpies of diffusing halogen ions in AgCl at medium and high temperatures and in AgBr at medium temperature, I propose a novel diffusion mechanism via cation divacancies. The activation enthalpies of the second diffusion mechanism of halogen ions in high-temperature AgBr show a q-dependence similar to that of the impurity cations. This finding indicates the presence of Schottky defects, in addition to the dominant Frenkel defects, in AgBr at high temperatures. The orbital-moments approach yields a Schottky enthalpy of 2.5 eV in AgBr and an estimate of more than 3 eV in AgCl.