NEW THERMALLY-INDUCED COLOR-CENTERS IN LITHIUM BORATE TUNGSTATE GLASSES, (LI2B4O7)(100-X)(WO3)(X)

A new color center based on a local structural and chemical change of the average berate tungstate glass matrix has been identified. The mat rices of the colored glasses all had a 1:1 ratio of Li2B4O7 and WO3 ex cept for glasses with x = 0. The reaction took place in air and at 120 0 K in all cases, while the reaction time varied between 10 and 60 min . Subsequent to reaction, the glasses were cooled to 550 K in 1 min an d then annealed in air at that temperature for 2 h. The coloration sta rts at a critical temperature T-c = 630 K and needs latent sites. The coloration process involves a distribution of energy barriers for the migration of Li+ with half-width comparable to its mean value, 2.91 eV . The achieved center concentrations are estimated to lie between 0.3 and 3 mol%. In addition, a color center has been found which is connec ted with Fe+3 impurities. This coloration is reversible and connected with a temperature-induced shift, 5 x 10(-4) eV/K, of the Fe-based osc illators.