Effect of aluminum on Ti-coordination in silicate glasses: A XANES study

The structure of glasses in the K2O-Al2O3-TiO2-SiO2 system was investigated using XANES spectroscopy. Glass samples, synthesized by quenching in air f rom high temperature fusions, represent the addition of Al2O3 to a base of composition K2TiSi4O11 in amounts corresponding to 0.25, 0.50, 0.75, 1.00, 1.25, and 1.50 mol p.f.u. In the Ti-free system, this range of alkali/alumi num ratios crosses the leucite stoichiometry at 1.0. Si K-edge and Al K-edg e spectra indicate tetrahedral environments for these elements, and show no variations related to coordination change as a function of Al content. Cha nges in the relative intensities of peaks in the Al K-edge, however, sugges t variation in the intertetrahedral (T-O-T) angle. We associate the decreas e of this angle for the glasses of peraluminous composition with the presen ce of triclusters of tetrahedra. The pre-edge peak absorption features in t he Ti K-edge XANES spectra indicate that the average Ti coordination decrea ses with the addition of Al2O3. We infer depletion of fivefold-coordinated titanium (possibly as alkali titanyl complexes), which are dominant in the Al-free glass, by the formation of fourfold coordinated Ti and alkali alumi nate complexes (up to a concentration of 40% in the most peraluminous glass ). Significant amounts of Ti-[V] remain present, even at peraluminous compo sitions, in further support of tricluster formation as a mechanism for Al i ncorporation.