The shallow electrons which were associated with certain impurities or interstitial Ag in these halides were thought to play key roles in some photo-induced processes. Theoretical calculations were performed by using a method which had been developed for treating excited electron centers in insulators. In both AgCl and AgBr, the Frenkel-pair energies were found to vary monotonically as a function of the vacancy-interstitial separation. This demonstrated the energetic tendency of the interstitial diffusion towards the vacancy. In order to reproduce the very small activation energy for interstitial Ag diffusion, a simple model for quadrupolar deformation of the Ag+ ion was used. It was found that the barrier to diffusion from the second cell to the vacancy was 0.4eV; compared with a much lower value (about 0.07eV) far from the vacancy. The role of the excited electron in Ag diffusion was studied, and the calculations showed that the electron could be bound to the Ag vacancy in a diffuse orbital when an interstitial Ag ion was nearby. The diffuse electron was found to reduce further the activation energy at moderate distances from the vacancy. The large barrier in the second cell from the vacancy was significantly reduced by the diffuse electron.

Role of the Excited Electron in the Diffusion of Interstitials in AgCl and AgBr. C.Fu, K.S.Song: Physical Review B, 1999, 59[4], 2529–36