Complex Formation and Photoreduction Processes in Sulphur Doped Silver Bromide

Abstract

The dark conductivity of annealed silver bromide single crystals, containing trace amounts of silver sulfide, has been measured as a function of dopant concentration. At 21 °C the conductivity exhibited a more rapid than linear increase with silver sulfide concentration at levels below 8 ppm. At 4 ppm, the lowest level achieved, a limiting dependence of the conductivity upon the fourth pwoer of the dopant concentration was observed. The room temeprature conductivity of the annealed samples became constant at dopant levels above 10 ppm. The dark conductivity of these samples, measured at 21°C after exposure to light at room temperature, dropped by more than an order of magnitude to constant levels typical of undoped silver bromide in the extrinsic range. This negative photoeffect could be largely reversed upon subsequent annealing. These results imply the formation of electrically active complexes incorporating several (~ 4) sulphur ions. The observed negative photoeffect is attributed to the photoreduction of interstitial silver ions to free silver. The relevance of the results to the photographic process is discussed.