Structure and Optical Absorption of AgI Microcrystals

Abstract

Large corrections to the x-ray intensities of the standard powder diffraction pattern of hexagonal AgI are proposed. Based on these intensities, it appears that some precipitates which have been described in the literature as pure hexagonal have actually contained about 30% of the cubic phase. Hexagonal AgI was produced during precipitation only by slow crystallization and without an excess of either ${\mathrm{Ag}}^{+}$ or ${\mathrm{I}}^{-}$. Faster precipitations gave mixtures of the cubic and hexagonal phases, and crystals whose average diameter was as small at 150 Å. The Rayleigh scattering of these suspensions was sufficiently small that the absorption coefficients of the crystals could be readily measured. The usually strong exciton absorption peak at about 420 mμ was missing in the small crystals. When the crystals were made to grow, the exciton peak appeared slightly below 420 mμ for crystals which became more hexagonal as they increased in size, and somewhat above 420 mμ for crystals which became more cubic. The decrease in absorption for AgI in the exciton region was much more than for the absorption edge of AgBr for crystals of comparable small size. Additional mechanisms for the loss of absorption in AgI compared with AgBr are apparently indicated. It is suggested that the exciton radius in AgI is so large that the exciton cannot be formed close to the surface.