A fine-scale nanostructure in $\gamma$-alumina

Despite the preeminent industrial importance of $\gamma $-alumina in catalysis, details of the structure remain unresolved due to its 15-30$\sim $nm domain nanocrystalline nature. Diffraction patterns are broad and single crystals are not available making accurate structural solution difficult using conventional crystallographic methods. We have applied a local structural technique, the atomic pair distribution function (PDF) analysis of powder diffraction, to obtain a quantitative structure. This is a total scattering technique that incorporates both Bragg and diffuse scattering information in the PDF, allowing all diffracted intensities from the XRD pattern to be equally considered. Surprisingly, we find a previously unknown fine-scale nanostructure with a domain size $\sim $ 1 nm. Within these nanodomains the oxygen sublattice is modified from the average structure and retains aspects of the boehmite precursor. This results in a novel and unexpected view of the $\gamma $-alumina structure since earlier controversies about it centered on the arrangement of Al ions among different cation sites, whereas the oxygen sublattice arrangement was not usually questioned.