SOL-GEL PROCESSING OF AN ENTIRELY NEW FAMILY OF MIXED-VALENCE GLASSES

Electro- and photochromic WO3-based glasses have been prepared by conventional melting. In this case, high concentrations of WO3 are required to induce the formation of larger complexes of HmWnO3n-m in the glasses and, therefore, the electro- or photochromic effect. The sol-gel process presents an alternative route for preparation of glasses, with low tungsten content. In low temperature syntheses, the greater kinetic control allows stabilization of intermediate states. The tungsten-doped silico-borate glasses obtained are monolithic, transparent and reversibly photochromic. The synthesis and characterization of these glasses are presented. The photochromic effect was investigated by absorption spectroscopy. The most attractive aspect of this method is the fact that the concentration of tungsten can be chosen and more importantly the size and the structure of the cluster, and therefore the optical properties, can be adjusted. Polyoxometalates as discrete oxides are isolated in a glass matrix and can be reduced.