Site Connectivities in Amorphous Materials Studied by Double-Resonance NMR of Quadrupolar Nuclei:  High-Resolution ¹¹B ↔ ²⁷Al Spectroscopy of Aluminoborate Glasses

Abstract

The utility of heteronuclear double-resonance NMR methods involving the quadrupolar nuclei 11 B and 27 Al 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 AlO 4/2 sites, the calcium- and magnesium-containing glasses also possess larger fractions of five- and six-coordinated aluminum sites. 27 Al{ 11 B} REDOR results indicate that all of these aluminum environments interact equally strongly with the dominant BO 3/2 sites in both glasses. In addition, the reverse 11 B{ 27 Al} REDOR results reveal that the extent of the AlO 4/2 −BO 4/2 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 Mg 2+ -based glasses tend toward a randomization of connectivities. Finally, two-dimensional heteronuclear correlation spectra obtained via 11 B{ 27 Al} CPMAS suggest that AlO 5 and AlO 6 sites can provide a mechanism for stabilizing tetrahedral BO 4/2 units.