Non-bridging oxygen sites in barium borosilicate glasses: results from B-11 and O-17 NMR

The concentration and structural role of non-bridging oxygen (NBO), defined as oxygen bonded to one framework cation, play a major role in the physica l properties of glasses and liquids. In this study, the distribution of NBO connected to either silicon and boron was investigated in a borosilicate g lass system using O-17 magic-angle spinning (MAS) and triple quantum (3QMAS ) nuclear magnetic resonance. In order to obtain better separation of diffe rent O-17 NBO resonances, barium borosilicate glasses were chosen and measu red by MAS and 3QMAS. The two-dimensional 3QMAS spectra show clear resoluti on between peaks for Si-NBO and B-NBO, allowing their relative proportions to be at least roughly quantified for the first time. The NMR parameters fo r these sites were also determined and are consistent with previous studies of binary barium berate and silicate systems. Site populations are signifi cantly different from predictions of conventional models based on alkali bo rosilicates, which suggests a more disordered nature for these barium boros ilicate systems. In order to explain the anomalously high NBO concentration s, we hypothesize that a few percent of the oxygens may have three network cation neighbors, forming 'triclusters'. (C) 2000 Elsevier Science B.V. All rights reserved.