Short-Range Order and Site Connectivities in Sodium Aluminoborate Glasses:  I. Quantification of Local Environments by High-Resolution 11B, 23Na, and 27Al Solid-State NMR

• Published in: The journal of physical chemistry. B Vol. 102; no. 23; pp. 4495 - 4506
• Main Authors: Züchner, Lars, Chan, Jerry C. C, Müller-Warmuth, Werner, Eckert, Hellmut
• Format: Journal Article
• Language: English; Japanese
• Published: American Chemical Society 04-06-1998
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp980587s

The local structure of glasses in the system Na2O−B2O3−Al2O3 is studied by multinuclear magic-angle spinning (MAS) solid-state NMR spectroscopy at 7.0 and 11.7 T. The 23Na isotropic chemical shifts vary linearly with Na2O concentration, indicating that the sodium ions are homogeneously distributed over the network and not clustered. The 11B MAS NMR spectra reveal the presence of trigonal BO3/2 units, tetrahedral BO4/2 - sites, and three-coordinate BO2/2O- species containing nonbridging oxygen. The quantitative contributions of these three types of sites are obtained by detailed computer simulations of these spectra. High-resolution 27Al satellite transition spectroscopy indicates that aluminum is predominantly present as AlO4/2 - sites. The structure of these glasses is discussed in terms of various melt reaction schemes, in which the network former oxides B2O3 and Al2O3 react with O2- introduced by the network modifier Na2O. While the data suggest that the formation of AlO4/2 - units takes priority over boron conversion, a detailed analysis of the oxide balance arising from these reactions indicates that small amounts of five- and six-coordinate aluminum must be present in nearly all of the samples; this conclusion is further confirmed by 27Al 2-D triple-quantum NMR experiments. Although these high-resolution solid-state NMR spectra primarily inform about nearest-neighbor environments, they also allow inferences about the connectivities of these sites. Thus, a detailed inspection of compositional isotropic chemical shift trends suggests that the framework sites present in these glasses are not interlinked statistically, but rather that the tetrahedral BO4/2 - and AlO4/2 - sites are preferentially surrounded by three-coordinate boron. On the basis of this concept, it is also possible to explain the compositional dependence of the glass transition temperature on a structural basis in terms of an average framework site connectivity.