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

Abstract

The local structure of glasses in the system Na 2 O−B 2 O 3 −Al 2 O 3 is studied by multinuclear magic-angle spinning (MAS) solid-state NMR spectroscopy at 7.0 and 11.7 T. The 23 Na isotropic chemical shifts vary linearly with Na 2 O concentration, indicating that the sodium ions are homogeneously distributed over the network and not clustered. The 11 B MAS NMR spectra reveal the presence of trigonal BO 3/2 units, tetrahedral BO 4/2 - sites, and three-coordinate BO 2/2 O - species containing nonbridging oxygen. The quantitative contributions of these three types of sites are obtained by detailed computer simulations of these spectra. High-resolution 27 Al satellite transition spectroscopy indicates that aluminum is predominantly present as AlO 4/2 - sites. The structure of these glasses is discussed in terms of various melt reaction schemes, in which the network former oxides B 2 O 3 and Al 2 O 3 react with O 2- introduced by the network modifier Na 2 O. While the data suggest that the formation of AlO 4/2 - 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 27 Al 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 BO 4/2 - and AlO 4/2 - 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.