Calculated and experimental defect parameters for silver halides

Abstract

Our present detailed knowledge of the defect structures of silver chloride and silver bromide results from extensive experimental measurements the interpretation of which has, in recent years, been assisted by calculated values for some defect parameters. Experimental data show that cation Frenkel defects predominate in the materials and that the silver ions move by two kinds of interstitial mechanism as well as through vacancy motion. Both conductance and diffusion measurements indicate that the concentration of Frenkel defects rises more sharply in the 150 K temperature interval below the melting points of these crystals than would be predicted by a temperature-independent defect-formation energy, even when long-range interactions between defects are taken into account. Atomistic simulations techniques, based on the Mott–Littleton approximation, have provided values for defect formation energies and, in the quasi-harmonic approximation, the temperature-dependence of the effect formation energy has been quantified in good agreement with experiment. They have also confirmed the importance of the deformation of the silver ion during its migration, and have provided insights into the motion and clustering of dopant ions. Currently three-body forces are being introduced into the modelling of the silver halides in new interionic potentials and progress in the calculation of defect energies using these potentials is reported.