Short and medium range order in sodium aluminoborate glasses.: 2.: Site connectivities and cation distributions studied by rotational echo double resonance NMR spectroscopy

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 Li6Al2(BO3)(4), NaBO2, and Na2B4O7 The heterodipolar multispin interaction between the quadrupolar spin systems Al-27 <-> B-11 and Na-23 <-> B-11 is analyzed in terms of approximate second moments, which are extracted from REDOR data measured at short dipolar evolution times. On the basis of Al-27{B-11} and B-11{Al-27}-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 BP3/2 and BO2/2O- units 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 Na-23{B-11}- and B-11{Na-23}-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.