11B NMR spectra and structure of boric oxide and alkali borate glasses

High-resolution 11B magic-angle-spinning (MAS) NMR investigations of boric oxide and alkali borate glasses have been carried out. The chemical shift of the trigonal boron shows anomalous behaviour around 10 mol. % alkali oxide. In an attempt to explain this unusual feature, we have carefully examined the structural model for B2O3 glass. The study suggests that around 66% of the boron atoms is likely to be present in the boroxol units, the rest being present in loose BO3/2 units. This model is not only consistent with the earlier literature but also shows that stringent topochemical factors are involved in the formation of the tetraborate and the diborate units in alkali borate glasses. It seems plausible that the remarkable tendency of B2O3 to vitrify may itself have a structural origin.