Non-bridging oxygen sites in barium borosilicate glasses: results from 11B and 17O 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 physical properties of glasses and liquids. In this study, the distribution of NBO connected to either silicon and boron was investigated in a borosilicate glass system using 17O magic-angle spinning (MAS) and triple quantum (3QMAS) nuclear magnetic resonance. In order to obtain better separation of different 17O NBO resonances, barium borosilicate glasses were chosen and measured by MAS and 3QMAS. The two-dimensional 3QMAS spectra show clear resolution 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 for these sites were also determined and are consistent with previous studies of binary barium borate and silicate systems. Site populations are significantly different from predictions of conventional models based on alkali borosilicates, which suggests a more disordered nature for these barium borosilicate systems. In order to explain the anomalously high NBO concentrations, we hypothesize that a few percent of the oxygens may have three network cation neighbors, forming ‘triclusters’.