SHORT-RANGE ORDER AND SITE CONNECTIVITIES IN SODIUM ALUMINOBORATE GLASSES - I - QUANTIFICATION OF LOCAL ENVIRONMENTS BY HIGH-RESOLUTION B-11, NA-23, AND AL-27 SOLID-STATE NMR

The local structure of glasses in the system Na2O-B2O3-Al2O3 is studie d by multinuclear magic-angle spinning (MAS) solid-state NMR spectrosc opy at 7.0 and 11.7 T. The Na-23 isotropic chemical shifts vary linear ly with Na2O concentration, indicating that the sodium ions are homoge neously distributed over the network and not clustered. The B-11 MAS N MR spectra reveal the presence of trigonal BO3/2 units, tetrahedral BO 4/2- sites, and three-coordinate BO2/2O- species containing nonbridgin g oxygen. The quantitative contributions of these three types of sites are obtained by detailed computer simulations of these spectra. High- resolution Al-27 Satellite transition spectroscopy indicates that alum inum is predominantly present as AlO4/2- sites. The structure of these glasses is discussed in terms of various melt reaction schemes, in wh ich the network former oxides B2O3 and Al2O3 react with O2- introduced by the network modifier Na2O. While the data suggest that the formati on of AlO4/2- units takes priority over boron conversion, a detailed a nalysis of the oxide balance arising from these reactions indicates th at small amounts of five-and six-coordinate aluminum must be present i n nearly all of the samples; this conclusion is further confirmed by A l-27 2-D triple-quantum NMR experiments. Although these high-resolutio n solid-state NMR spectra primarily inform about nearest-neighbor envi ronments, they also allow inferences about the connectivities of these sites. Thus, a detailed inspection of compositional isotropic chemica l shift trends suggests that the framework sites present in these glas ses are not interlinked statistically, but rather that the tetrahedral BO4/2- and AlO4/2- sites are preferentially surrounded by three-coord inate boron. On the basis of this concept, it is also possible to expl ain the compositional dependence of the glass transition temperature o n a structural basis in terms of an average framework site connectivit y.