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 11B and 27Al 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-45B2O3-30Al2O3 and 40M(2)O-40B2O3-20Al2O3 (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 tetrahe...