COMPUTATION OF THE ENERGETICS OF SURFACE VACANCY AND INTERSTITIAL GENERATION IN SILVER-HALIDE

We have considered alternative mechanisms through which flat, stepped, or kinked surfaces of silver halide may participate in the generation of interstitial or vacancy silver ions. In the energetically favored mechanism, the surface sites ionize to generate an adsorbed silver ion and a vacancy. The former has a smaller energy requirement for exchange into the bulk than the latter, as deduced from our calculations. Positive kink sites require the smallest overall energy for this generation step compared to flat {100} or stepped {105} surfaces. The nature of the space-charge layer involving surface negative charge and subsurface silver ions agrees well with the accepted experimental picture. Computed energies of these processes are presented along with a preliminary estimate of the entropy of formation of point defects from the AgBr surface. A calibration of our procedure for NaCl surfaces is made by comparison of our calculated energies of formation to experimental enthalpy values for vacancies and shows excellent agreement.