Site connectivities in amorphous materials studied by double-resonance NMR of quadrupolar nuclei:: High-resolution 11B ⇆ 27Al spectroscopy of aluminoborate glasses

The utility of heteronuclear double-resonance NMR methods involving the quadrupolar nuclei B-11 and Al-27 is demonstrated for probing the;intermediate-range order in aluminoborate glasses. Results from both rotational echo double resonance (REDOR) and heteronuclear cross-polarization magic angle spinning (CPMAS) studies are presented and discussed. While both techniques are strongly affected by the presence of large quadrupolar splittings, a working strategy is developed on the basis of which reliable structural interpretations are possible. Using this strategy, the effects of the cation type M on the structures of two basic glass compositions, 25M((2))O-45B(2)O(3)-30Al(2)O(3) and 40M((2))O-40B(2)O(3)-20Al(2)O(3) (M = Na, Ca, Mg), are discussed. Within each series having the same basic network composition, replacement of the modifier cation Na by Ca or Mg produces a large change in the speciation of framework aluminum ("cation effect"): while in the sodium-containing glass the overwhelming fraction of Al is present as tetrahedral AlO4/2 sites, the calcium- and magnesium-containing glasses also possess larger fractions of five- and six-coordinated aluminum sites. Al-27(B-11) REDOR results indicate that all of these aluminum environments interact equally strongly with the dominant BO3/2 sites in both glasses. In addition, the reverse B-11{Al-27} REDOR results reveal that the extent of the AlO4/2-BO4/2 interaction in the network is highly dependent on the framework composition and on the M cation type. While sodium aluminoborate glasses reveal strong chemical ordering (avoidance of intertetrahedral linkages), the analogous Mg2+-based glasses tend toward a randomization of connectivities. Finally, two-dimensional heteronuclear correlation spectra obtained via B-11(Al-27) CPMAS suggest that AlO5 and AlO6 sites can provide a mechanism for stabilizing tetrahedral BO4/2 units.