Non-bridging oxygens in borate glasses: characterization by B-11 and O-17 MAS and 3QMAS NMR

The concentrations of non-bridging oxygens (NBO) in oxide glasses has major effects on their properties and on those of their precursor glass melts. I n berate and borosilicate glasses, the presence of NBO bonded to boron has generally been inferred from B-11 NMR spectra and mass balance consideratio ns. Here we report the direct observation of such NBO using O-17 MAS and 3Q MAS techniques, and compare estimates of their populations with those deriv ed from high-resolution B-11 MAS spectra. For the latter, two independent m ethods are used, based on the ratios of trigonal to tetrahedral boron and o n the concentrations of trigonal boron sites with large quadrupolar asymmet ry parameters. We include data on crystalline sodium pyroborate (Na4B2O5) a nd sodium metaborate (NaBO2), and several sodium and barium berate glasses. O-17 chemical shifts and quadrupolar coupling constants for NBO bonded to boron vary considerably depending on their coordination environment. In bor osilicates, peaks for this species may be hidden by overlap with B-O-Si or Si-O-Si resonance. (C) 2000 Elsevier Science B.V. All rights reserved.