29Si MAS-NMR Study of the Short-Range Order in Lithium Borosilicate Glasses

ABSTRACT

²⁹Si MAS-NMR measurements have been made on a series of lithium borosilicate glasses of general composition RLi₂O.B₂O₃·KSiO₂. At low alkali contents (R < 1), the ²⁹Si resonance envelope is broadened and indicates a distribution of Si sites. As R increases above 1, the FWHM of the ²⁹Si resonance narrows considerably to that representative of a single chemical site. Simultaneously, the average chemical shift of the resonance shifts upfield in agreement with the trends found in the binary lithium silicate glass system. Using the chemical shifts for the individual Q species in the binary system it was found that very good agreement between the chemical shifts of the binary glasses and the ternary glasses examined here could be achieved if a model of proportional sharing of the added oxygen (from lithia) between silicate and borate units was used. In contrast to the ¹¹B NMR studies of these same glasses, the ²⁹Si NMR data are quantitatively best-fit if it is assumed that the proportional sharing of the oxygen from the added lithia begins at R= 0. Models of sharing developed from the ¹¹B NMR studies of these glasses, where proportional sharing above a certain fixed (independent of K) or variable (dependent on K) minimum R₀, have been reexamined and were quantitatively shown through residual analysis to give consistently poorer fits to our data. At present the reasons for the discrepancy between the two sets of NMR data are unknown.