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Characterization of the behaviour of titanium in silicate melts is of extreme importance both in the glass industry and in igneous petrology because of the very strong effect of even very small amounts of titanium on the physical and chemical properties of glasses. Given the relationship between structure and properties of silicate melts, an intensive effort has been taken to characterize the structure of Ti-bearing silicate glasses in order to shed light into the interpretation of the variation on physical and chemical properties in both natural and synthetic systems. In this project, we decided to investigate the structure of glasses in the M₂O (MO)-Al₂O₃-TiO₂ system, where M is an alkaline or an alkaline earth cation, by using XANES spectroscopy. Here we report results obtained for the system K₂O-Al₂O₃-TiO₂-SiO₂. Preliminary results on the system BaO-Al₂O₃-TiO₂ have also been obtained but they will not be discussed here. Glass samples have been synthesized by quenching in air from high temperature fusions. The samples investigated represent the addition of Al₂O₃ to a base of composition K₂TiSi₄O₁₁ in amounts corresponding to mol. p.f.u. = 0.25, 0.50, 0.75, 1.00, 1.25, 1.50. This range of alkali/aluminum ratios crosses the leucite stoichiometry at 1.0. The Ti K-edge XANES spectra were measured at LURE (France) using the XAS2 line equipped with a Si (111) monochromator. Experiments at the Al and Si K-edge were carried out at SSRL (Stanford, California) with the SPEAR storage ring operating at energy 3 GeV and injection current of 100 mA. We operated on beam line SBO3-3 equipped with JUMBO monochromator of the double crystal type, which were two plates of a YB66 crystal cut along the (400) plane.

The XANES spectra exhibit different trends with increasing Al₂O₃ content. The aluminum and silicon K-edges do not seem to show any visible changes as a function of Al content. Spectra recorded on the Ti K-edge show, however, a distinct variation of the spectral features (pre-edge peak P, peak A and peak B) as a function of increasing Al content, suggesting a change in local geometry around the Ti absorber (Fig. 3.6-13). With increasing Al₂O₃ content, peak P increases in intensity and shifts to lower energy, peak A decreases in intensity with a minimum at Al₂O₃ = 0.75% and then increases again slightly, peak B clearly shifts toward higher energy (+ 5 eV). As demonstrated in our previous work, the pre-edge peak intensity is correlated with titanium coordination number in glasses. Variations of XANES features as a function of aluminum content can be interpreted therefore to suggest a variation of polyhedral geometry around titanium. To infer the average coordination number of titanium on our glasses we applied the correlation curve between the peak intensity and coordination number as calculated in our previous studies. From this analysis, we calculated that the average CN increases from the Al-free tetrasilicate composition up to 0.75 mol p.f.u. whereas further addition of Al₂O₃, up to 1.25 mol p.f.u., decreases the average CN. These results can be interpreted as to indicate a competition between different titanate complexes in 4-, 5-, and 6-coordination as a function of chemical composition in the system.
 

Fig. 3.6-13: Experimental XANES spectra of Ti K-edge in K2O-Al2O3-TiO2-SiO2 glasses varying in Al2O3 content.

We performed 1 atm density measurements on the same samples in order to detect any correlations between physical properties and structure in these systems. These results are shown in Fig. 3.6-14. The density decreases for the peralkaline composition and reaches a minimum for the composition where Al₂O₃/K₂O=1 and then increases again for peraluminous samples. Several possible explanations can be envisaged to interpret both the XANES and density data. The density minimum could result from the combined effects of aluminum substitution for potassium together with an increase in the stabilization of five-coordinated Ti polyhedra. XANES simulations utilizing one-electron multiple scattering (MS) theory to simulate the local Ti environment in a glass as well as additional measurements to investigate the effect of different cations in the structure will be carried out in the near future.
 

Fig. 3.6-14: Densities of K2O-Al2O3-TiO2-SiO2 glasses and their variation as a function of Al2O3 content.