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 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 bor...