The photochromism of Ag2S-HgI2

• Main Author: McDonnell, N. P. H.
• Published: University of Bath 1969
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.767473

A review of previous work on photochromic compounds is given with emphasis upon those inorganic compounds showing photochromism in the solid state. Attention is drawn to the behaviour of photochromic and non-photochromic compounds similar to Ag2S-HgI2 reaction products. Detailed studies of the methods required for the preparation of Ag2S-HgI2 products are described and the influence of the vapour phase is investigated. Measurements of the reaction products using X-ray powder diffraction, diffuse reflectance techniques and measurements of electrical conductivity indicate that they are compounds having an unstable defect structure below 50 mole per cent Ag2S and that the products are ionic conductors. The effect of heat on the products is investigated in some detail and a model for the Ag2S-Hgl2 reaction processes is proposed in the light of present knowledge of powder reactions. Transmission and reflectance studies on powders show a correlation between the bleaching and colouring kinetics and the composition of the Ag2S-HgI2 products. An explanation for this behaviour is given using theories of vacancy controlled diffusion processes.