Fig. 8

Simulated ²⁹Si NMR spectra for Ti-bearing Na-silicate glasses, generated using Gaussian peaks at the positions and shapes of the Ti-free a NS4 and b NS8 glasses, by varying the proportions of Q⁴ (Ti = 1) and Q⁴ (Ti = 0) peaks. The peaks that compose these model spectra at 40% Q⁴ (Ti = 1) and 60% Q⁴ (Ti = 0) are shown in 8 c and 8 d. Increasing Ti content in the simulations is represented by the growth of a third Q⁴ peak at a higher frequency as a result of nearest-neighbor interactions with ^[4]Ti nearest neighbors. The simulations are similar to the actual spectra, with the notable difference that the simulations over-estimate the amount of Q⁴ Si with ^[4]Ti nearest neighbors Q⁴ (Ti = 1). e, f If Ti was incorporated entirely by the Q⁴ (Ti = 1) mechanism, then observed spectra would be reflective of 100% Q⁴ (Ti = 1) at high Ti content in NS4 and NS8 glasses. However, NS8 glass with 15 mol% TiO2 and NS4 glass with 10 and 15% TiO2 which would have all Si in Q⁴ (Ti = 1) at these concentrations are still significantly far from Q⁴ (Ti = 1). This over-estimation suggests that Ti is clustering at relatively low bulk TiO₂, an observation that is supported by the Ti XAFS observations of significant ^[5]Ti in the NS8 glasses