A quantitative NMR strategy is developed for the study of the network connectivity (intermediate range structure) and the cation distribution in the sodium aluminoborate glass system. The strategy is based on the analysis of rotational echo double resonance (REDOR) spectroscopy applied to the glasses and the crystalline model compounds Li 6 Al 2 (BO 3 ) 4 , NaBO 2 , and Na 2 B 4 O 7 . The heterodipolar multispin interaction between the quadrupolar spin systems 27 Al T 11 B and 23 Na T 11 B is analyzed in terms of approximate second moments, which are extracted from REDOR data measured at short dipolar evolution times. On the basis of 27 Al{ 11 B} and 11 B{ 27 Al}-REDOR results, the framework connectivity distribution is extracted and compared to statistical and preferential bonding distribution scenarios, respectively. The sodium aluminoborate system is characterized by a large degree of bonding regularity in the framework. Four-coordinated aluminum is preferentially linked to BO 3/2 and BO 2/2 Ounits and vice versa, while linking between two four-coordinate units is disfavored, as expected on the basis of bond valence considerations. In glasses with low aluminum contents and/or high sodium contents, there is a greater tendency toward a randomization of connectivities. Spatial correlations between the sodium nuclei and the boron atoms in the network are probed by complementary 23 Na{ 11 B}-and 11 B{ 23 Na}-REDOR experiments. The results reveal that both the three-and the four-coordinated boron species interact equally strongly with sodium cations, consistent with an overall homogeneous cation distribution.

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.

Modern solid state nuclear magnetic resonance (NMR) techniques afford powerful experimental strategies for elucidating detailed structural information in noncrystalline materials. While previous research emphasis has been on the identification and quantification of local environments (short-range-order), primarily by using the technique of magic-angle spinning (MAS) NMR, correlations beyond the nearest neighbor coordination sphere (intermediate-range order) can be studied by more sophisticated methods. This contribution focuses on recent advances made for a variety of oxide glasses, using methods of dipolar spectroscopy: Connectivities among the structural building blocks present in sodium aluminoborate glasses have been exposed using 11B[27Al] and 27Al{11 B} rotational echo double resonance (REDOR) NMR. Similarly, 29Si{31P] and 29Si[7Li) REDOR data form the basis of a structural model integrating six-coordinated silicon into the network structure of alkali silicophosphate glasses. The spatial cation distributions in sodium silicate glasses are studied by 23Na spin echo decay spectroscopy, offering experimental evidence for inhomogeneous cation distributions at low alkaline contents. Finally, for mixed alkali sodium lithium silicate glasses, 23Na{7LiJ spin echo double resonance (SEDOR) and 29Si[23Na} and 29Si[7Li} REDOR spectroscopies provide important experimental criteria for testing hypothesized relative Na-Li cation ordering scenarios.

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