Structural aspects of mullite-type NaAl9O14 studied by Al-27 and Na-23 solid-state MAS and DOR NMR techniques

Na-23 and Al-27 MAS and DOR NMR spectra are reported for the mullite-type crystalline compound NaAl9O14 and its amorphous precursor using a range of applied magnetic fields (8.45-16.9 T). The crystalline compound is shown by Al-27 NMR to contain aluminium in one octahedral and two tetrahedral sites, a distribution consistent with expectations based on a recent Rietveld structure determination. By contrast, two approximately equally populated non-identical Na sites can be distinguished by Na-23 NMR. Spectral simulation indicates these sites to be of slightly different distortion but of similar isotropic chemical shift. This result is not consistent with the average Pbam structure deduced from Rietveld X-ray analysis, in which the two possible Na sites are symmetrically equivalent. These measurements illustrate the ability of NMR techniques to distinguish small variations in atomic environment which are not resolved in the averaged X-ray structure.