Structure of alkali (alumino)silicate glasses. I. Tl⁺ luminescence and the nonbridging oxygen issue

Optical excitation spectra of Tl⁺ ions, corresponding to emission at 350 nm, have been measured in Na aluminosilicate glasses. These excitation spectra were found to be good representations of absorption spectra. The excitation spectra are shown to be superpositions of two primary spectra, which are identified with Tl⁺ acting as network modifiers or as charge compensators for network aluminums. When A1/Na ≥ 1, only the charge compensator spectrum can be observed. As A1/Na decreases below unity, the fraction of the charge compensator spectrum decreases rapidly, and the fraction of the network modifier spectrum increases correspondingly. No other discontinuous behavior in the characteristics of the excitation spectra as a function of composition was found. These results strongly support the traditional model of alkali aluminosilicate structure, in which the critical compositions for (dis)appearance of nonbridging oxygens are given by A1/Na = 1; they contradict reports of XPS measurements from which it had been concluded that the critical compositions are given by A1/Na ≅ 0.7. Measurements of T_g also contradict the conclusions drawn from those XPS measurements. The rapid appearance of the network modifier spectrum immediately below A1/Na = 1 is attributed to Tl⁺ ions being more likely to occupy network modifier sites than charge compensator sites. A statistical mechanical calculation, used in conjunction with the excitation spectra, shows that the preference energy favoring Tl⁺ occupation of n network modifier sites is ≅ 750 cm^-1.