Location of the Spinel Vacancies in γ-Al2 O3.

A non-spinel model for the structure of γ-Al2 O3 , with 25 % of the Al3+ cations at tetrahedral positions, has been the subject of wide interest. However, 17 O NMR measurements and, more recently, 27 Al NMR measurements have shown that there are considerably more Al3+ cations at tetrahedral positions. This means that the Al3+ vacancies in γ-Al2 O3 are not at tetrahedral but at octahedral positions, as in isostructural γ-Fe2 O3 and in accordance with density functional theory predictions. This has consequences with regard to the surface structure of γ-Al2 O3 , and thus, for catalysis.