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Abstract

We have prepared under ultrahigh vacuum conditions a NiAl(110) wedge which was oxidized such that, after oxidation, small support-free Al₂O₃ crystallites were formed at the edge of the wedge, and the remaining part of the wedge was covered by a thin well-ordered Al₂O₃ film. This film, which has been characterized in great detail in earlier studies [J. Libuda et al., Surf. Sci. 318 (1994) 61], was transferred in air to a transmission electron microscope (TEM) and studied with respect to its geometric structure. The particular sample preparation allows for a direct comparison between the structure of the film and the bulk aluminum oxide. We find that the supported oxide exhibits a γ-Al₂O₃-like structure similar to that observed previously under UHV conditions, indicating that exposure to air has only a limited influence on the film. The lattice constant of the oxide film agrees within ∼2% with that reported for bulk Al₂O₃. The domain structure of the alumina film as found in the ultrahigh vacuum experiments was also identified using TEM of the supported layer. The support-free oxide formed at the edge of the wedge exhibits the structure of the supported film formed under UHV conditions with deviations of the lattice constants of ∼7% in one direction and ∼1% in the other.