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

摘要

The local structure of glasses in the system Na_2O-B_2O_3-Al_2O_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_2O 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_2O_3 and Al_2O_3 react with O~(2-) introduced by the network modifier Na_2O. 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 environment, 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 the 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.