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

Abstract

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 behavior around 10 mol.% alkali oxide. In an attempt to explain this unusual feature, the structural model for B₃O₃ glass was examd. The study suggests that around 66% of the B atoms is likely to be present in the boroxol units, the rest being present in loose BO_3/2 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 vitrifying may itself have a structural origin.