Fine-Scale Nanostructure in γ-Al₂O₃

We have applied a local structural technique, atomic pair distribution function (PDF) analysis of powder diffraction data to γ-Al 2 O 3, to obtain a quantitative structure. Refinements of the PDF support previous findings that nonspinel model representations are more suitable to describe the average structure of γ-Al 2 O 3, as opposed to the spinel model. Surprisingly, also we find a previously unknown fine-scale nanostructure with a domain size ∼ 1 nm. Modeling suggests that within these nanodomains the oxygen sublattice is modified from the average structure and retains aspects of the boehmite precursor that reflects a stacking fault in the γ-Al 2 O 3 matrix. This results in a novel and unexpected view of the γ-Al 2 O 3 structure because earlier controversies about it centered on the arrangement of Al ions among different cation sites, whereas the oxygen sublattice arrangement was rarely questioned. This average oxygen sublattice structure is recovered in our models on longer length scales by introducing aperiodic arrays of stacking faults. The use of γ-Al 2 O 3 in catalysis depends sensitively on its nanoporosity and defect structure. Here we present a new view which may allow for additional understanding and optimization of its functional properties.