ABSTRACT
Photochromic glasses doped with silver and copper halides exemplify heterogeneous materials. They represent a two-phase system consisting of a vitreous phase of host glass and of a finely dispersed photosensitive phase. Photochemical processes would be completely reversible. It is reasonable to believe that inorganic glasses can serve as such rigid hosts. Indeed, they have very high viscosity and low gas permeability at room temperature. The leakage of reaction products from the photosensitive phase to the host glass is prevented. An additional isolation of photoinduced reaction in a two-phase system is provided by the low mobility of electrons and holes in inorganic glasses. The range of heterogeneous photochromic glasses may be fairly wide as there is a great number of known photosensitive crystals. However, after long wavelength exposure when the photosensitive phase is excited and color centers are generated in microcrystals only, no stresses were obtained.
