Phototrogy of Alkaline Earth Tungstates Doped with Bismuth

Calcium, strontium, and barium tungstates and their solid solutions, doped with a small amount of bismuth, color remarkably on exposure to ultraviolet light and return to the original white by thermal fading or optical bleaching. The factors affecting the darkening and fading processes were studied experimentally. Under the same irradiation conditions the intensity of uv-induced coloration depended on the Bi content and preparation temperature of the tungstates. Bi contents higher than 0.1 molyo and preparation temperatures higher than 1250°C gave strong coloration. The rate of fading of uv-induced color increased with increasing Bi content. The fading was accelerated by heating; almost all the uv-induced color disappeared at 230°C in 30 min. The fading rate was also increased by visible light exposure. The uv-induced absorption spectra varied systematically with the composition of the tungstates.