Role of Dislocations in Silver Halide Photography

Abstract

Dislocations and donor centers have important functions in the optimization of the performance of silver halide emulsion grains. The basic properties of dislocations relevant to photographic sensitivity together with the experimental observations which established these properties are reviewed in this paper. Internal latent image and internal particles of photolytic silver are formed during exposure of AgCl and AgBr crystals, sensitized with Ag2O and Ag2 donor centers, by the separation of Ag atoms along the dislocation lines. The surface sites of termination of dislocations have enhanced reactivity compared with low-energy surfaces. Dissolution, chemical sensitization, and the initiation of surface chemical development occur at higher rates at these sites. These properties led to the concept of the dislocation sensitivity center. Development centers are formed by the combination of Ag atoms with Ag2 latent image growth nucleus precursors at the surfaces of dislocation sensitivity centers. The formation of Ag2 molecules along the subsurface dislocation of the center has to be minimized. Microcrystals of AgCl and AgBr with dislocations introduced during nucleation and initial growth pass into solution at a high rate during a short period of ripening leaving dislocation-free growth nuclei. For stable distributions of dislocations in microcrystals, concentration gradients of halide ions have to be established in heterogeneous crystals. Controlled densities of dislocations are introduced to reduce the elastic strains associated with steep concentration gradients. In double-structure grains with a higher iodide shell and in triple-structure grains with a higher iodide narrow zone, the concentration gradients can be established by the addition of a fine dispersion of Ag(Br,I) particles or of an iodide ion releasing compound. The resulting dislocation distributions are stable because of the immobility of the iodide ions in the crystal. The mechanisms involved in the creation of dislocations in structured tabular microcrystals are discussed.