Ionic and Electronic Properties of Silver Iodide Grains

Abstract

It has been observed in the preceding paper that the ionic conductivity of AgI grains with interstitial silver ions as carriers is comparable to that of octahedral AgBr ones. Since the activation energy of the migration of interstitial silver ions in AgI is much smaller than that in AgBr, it is estimated that the concentration of interstitial silver ions acting as shallow electron traps is much larger in AgI grains than in AgBr ones, providing an explanation for the observation by a microwave photoconductivity method that the photoconductivity with photoelectrons as carriers is smaller in AgI grains than in AgBr grains. The observation that latent image centers on AgI grains is oxidized at more negative redox potential than that on AgBr grains has been ascribed to the fact that the concentration of silver ions in the vicinity of the former is smaller than that in the vicinity of the latter owing to the difference in the solubility in water between them. A radiofrequency photoconductivity method has revealed that the activity of positive holes in AgI grains in an emulsion is higher than that in AgBr grains in an emulsion, and is depressed by increasing the concentration of silver ions in the emulsion. Although these results make it more difficult to form latent image centers on AgI grains than on AgBr ones in conventional photographic materials, the latent image formation on AgI grains could be enhanced by increasing the concentration of silver ions in conventional materials, and by using them in photothermographic materials, in which the concentration of silver ions was usually much higher than in conventional ones owing to the presence of silver carboxylate. Recent discovery that AgI grains are fixed during thermal development in photothermographic materials has merged with the above-stated results to reveal that AgI grains are suitable and useful for photothermographic materials.