The Photochromism of Ag2S-HgI2

Abstract
The bleaching kinetics of the photochromic material formed by the solid state reaction between silver sulphide and mercuric iodide at a molar ratio of 1: 4 have been studied. The thermal bleaching followed the law: where n is the concentration of coloured systems at time i, e is an activation energy term for the bleaching, A 0 and b are constants. Preliminary measurements indicate that ∈≃0·56 ev. It is suggested that the bleaching depends upon the cation vacancy concentration in the material, with silver ions diffusing via these vacancies from regions of high concentration near the surface into the bulk of the material.

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