The local environment of titanium in glasses ofthe system TiO_2-BaO-ZnO-B_2O_3 was studied by x-rayabsorption spectroscopy. In all glasses, for TiO_2 contentvarying from 35 to 49 mol percent, the titanium atoms aremainly fourfold coordinated network formers. For the glass withthe highest titanium content, about 20 percent of the titaniumenters into a sixfold environment, indicating the presence ofprecrystalline nuclei. These nuclei are responsible for thedecrease of the optical performance of the glass above thresholdtitanium content, prior to any observable crystallization.
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