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

High-resoln. 11B magic-angle-spinning (MAS) NMR investigations of boric oxide and alkali borate glasses have been carried out. The chem. shift of the trigonal boron shows anomalous behavior .apprx.10 mol.% alkali oxide. In an attempt to explain this unusual feature, the structural model for B3O3 glass was examd. The study suggests that at .apprx.66% of the B 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 topochem. 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