Structure of alkali (alumino) silicate glasses. II. Luminescence of thallium doped sodium aluminosilicates, and implications for optical basicity theories

Optical excitation and emission spectra are reported for Na aluminosilicate glassed doped with 0.001-0.005 M Tl⁺ ions, whose spectra probe Na⁺ environments in the glasses. The experimental data buttress earlier work by the authors, which showed that Al/Na = 1 is the condition for (dis)appearance of nonbridging oxygens in Na aluminosilicate glasses. The Tl⁺ spectra are in qualitative disagreement with theories of average optical basicity propounded by Jørgensen and by Duffy and Ingram (glass compositions were chosen so samples had theoretical optical basicities of Λ_th = 0.55, 0.57, or 0.60). In contrast to the single absorption band predicted by average basicity theories, two absorption bands were found for Tl⁺ in Na aluminosilicate glasses. The two bands arise from Tl⁺ acting as network modifiers and from Tl⁺ acting as charge compensators for network Al. The principal features of these spectra are shown to be consistent with XPS results. The assumptions underlying average and group optical basicity theories are critically reviewed and are found to be deficient in that: (1) they direct attention to a network atom (Si or Al) and the oxygen bonded to it, rather than to the luminescent probe ion and its neighboring oxygens; (2) they neglect lattice relaxation (Franck-Condon shift) effects. Important special circumstances are nevertheless identified in which average optical basicity theories may be useful guides to the peak energies of probe ion spectra.