Role of the excited electron in the diffusion of interstitials in AgCl and AgBr

Abstract

The shallow electrons associated with certain impurities or interstitial silver in AgBr and AgCl are believed to play key roles in some of the photoinduced processes. We have performed theoretical calculations on these systems using a method developed earlier for excited electron centers in insulators. In both AgCl and AgBr crystals the Frenkel-pair energies are found to vary monotonously as a function of the vacancy-interstitial separation, which shows the energetical tendency of the interstitial diffusion toward the vacancy. To obtain the very small activation energy of the interstitial silver diffusion, a simple model of quadrupolar deformation of the ${\mathrm{Ag}}^{+}$ ion is employed. We found that the barrier for diffusion from the second cell to the vacancy is substantial (e.g., 0.4 eV) compared with a much smaller value far from the vacancy, $\ensuremath{\approx}0.07 \mathrm{eV}.$ The role of the excited electron in silver diffusion is studied. Our calculation shows that the electron can be bound to the silver vacancy in a diffuse orbital when an interstitial silver ion is present nearby. The diffuse electron is found to further reduce the activation energy at moderate distance from the vacancy. Moreover, the large barrier in the second cell from the vacancy was significantly reduced by the diffuse electron. Possible mechanisms for the recently observed anomalous heat generation in photoexcited silver halides are presented.