Space charge characteristics of silver halide microcrystals

Abstract

A comparative study between the space charge characteristics of (100) and (111) AgX micro-crystals as a function of the grain size, is presented. The ionic conductivity of the silver halide micro-crystals as a function of temperature is determined by dielectric loss measurements on photographic emulsions. It is shown that the AgCl (100), AgBr (100), and AgBr (111) microcrystals have a net negative surface charge which is compensated by a positive space charge region formed by an excess of interstitial silver ions. It is experimentally proved that there exists an essential difference between the space charge characteristics of cubic and octahedral grains. For the AgCl (100) and AgBr (100) microcrystals an increase of the ionic conductivity is obtained but no change in activation energy for ionic conductivity when the grain size decreases. However, for the AgBr (111) microcrystals besides an increase of the ionic conductivity a decrease of the activation energy and the surface potential, with decreasing grain size is found. Both results are explained on basis of a previous reported space charge model for thin AgX films. The results of (100) microcrystals are supposed to be due to the overlapping of the space charge regions of opposite (100) planes. The change in the space charge characteristics of (111) AgBr surfaces with decreasing grain size, is ascribed to the limitation of the total number of available surface sites for Frenkel defect formation.