Crystalline aluminium borates with the mullite structure : A B-11 and Al-27 solid-state NMR study

Al-27 magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were acquired at 8.45, 14.1 and 16.45 T for a series of aluminium borates with the mullite structure (Al6-x B (x) O-9, where x has nominal values of 1 to 4) augmented with Al-27 multiple-quantum MAS NMR spectra at 8.45 T. Even though the Al-27 NMR spectra are complex, simulation of the combined set of data produced a relatively well-defined set of parameters (e.g., quadrupolar interaction, isotropic chemical shift, etc.) for each site. The B-11 MAS NMR spectra of the same compounds were also acquired at 14.1 T. Linear changes in the X-ray a-, b- and c-cell parameters with composition suggest that these compounds constitute a continuous series. Based on a Rietveld structural refinement of the compound synthesized as Al4B2O9, the resulting site occupancies and relative site distortions allow the identification of particular sites with specific NM resonances. Changes in the Al-27 and B-11 MAS NMR spectra of the related compounds with x = 1-4 show at the lowest Al contents a greater degree of asymmetry in the Al sites of the octahedral chains. A fairly distorted cross-linking tetrahedral site, which persists throughout the composition range, is accompanied in the lower Al compositions by two 5-fold coordinated Al-O units which are replaced by two more-regular tetrahedral Al-O sites as the Al content increases. In the compounds of lowest Al composition (i.e., highest B content) both the tetrahedral and trigonal cross-linking sites are distinguishable, but as the Al content increases, the BO4 units progressively disappear