X-ray absorption study of the coordination of titanium in sodium-titanium-silicate glasses

The local structure around titanium in sodium-titanium-silicate glasses was determined as a function of the silica content and the Na/Ti ratio. The samples covering the range 16.7-33 mol% Na2O and 16.7-33 mol% TiO2 were prepared by melting predetermined mixtures of Na2CO3, TiO2, and SiO2 at 1400 degrees C. They were studied using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies around the Ti K-edge at 4966 eV. The data indicate that titanium is found in two sites in the glass. In one site, titanium is coordinated by six oxygens; in the other, titanium is coordinated by five oxygens. The number of five-coordinated titanium atoms increases relative to the number of six-coordinated titanium atoms as the silica content of the glass decreases. The ratio of five- to six-coordinated sites decreases as the Na/Ti ratio of the glass decreases at a fixed silica content.