ROLE OF DISLOCATIONS IN SILVER-HALIDE PHOTOGRAPHY

Dislocations and donor centers have important functions in the optimiz ation of the performance of silver halide emulsion grains. The basic p roperties of dislocations relevant to photographic sensitivity togethe r with the experimental observations which established these propertie s are reviewed in this paper. Internal latent image and internal parti cles of photolytic silver are formed during exposure of AgCl and AgBr crystals, sensitized with Ag2O and Ag-2 donor centers, by the separati on of Ag atoms along the dislocation lines. The surface sites of termi nation of dislocations have enhanced reactivity compared with low-ener gy surfaces. Dissolution, chemical sensitization, and the initiation o f surface chemical development occur at higher rates at these sites. T hese properties led to the concept of the dislocation sensitivity cent er. Development centers are formed by the combination of Ag atoms with Ag-2 latent image growth nucleus precursors at the surfaces of disloc ation sensitivity centers. The formation of Ag-2 molecules along the s ubsurface dislocation of the center has to be minimized. Microcrystals of AgCl and AgBr with dislocations introduced during nucleation and i nitial growth pass into solution at a high rate during a short period of ripening leaving dislocation-free growth nuclei. For stable distrib utions of dislocations in microcrystals, concentration gradients of ha lide ions have to be established in heterogeneous crystals. Controlled densities of dislocations are introduced to reduce the elastic strain s associated with steep concentration gradients. In double-structure g rains with a higher iodide shell and in triple-structure grains with a higher iodide narrow zone, the concentration gradients can be establi shed by the addition of a fine dispersion of Ag(Br,I) particles or of an iodide ion releasing compound. The resulting dislocation distributi ons are stable because of the immobility of the iodide ions in the cry stal. The mechanisms involved in the creation of dislocations in struc tured tabular microcrystals are discussed.