| Abstract
| - A quantitative NMR strategy is developed for the study of the network connectivity (intermediate rangestructure) and the cation distribution in the sodium aluminoborate glass system. The strategy is based on theanalysis of rotational echo double resonance (REDOR) spectroscopy applied to the glasses and the crystallinemodel compounds Li6Al2(BO3)4, NaBO2, and Na2B4O7. The heterodipolar multispin interaction between thequadrupolar spin systems 27Al ↔ 11B and 23Na ↔ 11B is analyzed in terms of approximate second moments,which are extracted from REDOR data measured at short dipolar evolution times. On the basis of 27Al{11B}and 11B{27Al}-REDOR results, the framework connectivity distribution is extracted and compared to statisticaland preferential bonding distribution scenarios, respectively. The sodium aluminoborate system is characterizedby a large degree of bonding regularity in the framework. Four-coordinated aluminum is preferentially linkedto BO3/2 and BO2/2O- units and vice versa, while linking between two four-coordinate units is disfavored, asexpected on the basis of bond valence considerations. In glasses with low aluminum contents and/or highsodium contents, there is a greater tendency toward a randomization of connectivities. Spatial correlationsbetween the sodium nuclei and the boron atoms in the network are probed by complementary 23Na{11B}- and11B{23Na}-REDOR experiments. The results reveal that both the three- and the four-coordinated boron speciesinteract equally strongly with sodium cations, consistent with an overall homogeneous cation distribution.
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