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We have investigated the structure of the clean and the oxidized (111) surface of a Cu-Al alloy with 9 atomic percent Al by scanning tunnelling microscopy (STM), Auger electron spectroscopy (AES), low energy ion scattering (LEIS) and low energy electron diffraction (LEED). Annealing of the clean crystal to 680 °C leads to segregation of Al to the surface. The Al concentration at the annealed surface is (23 ± 2)% and domains with a (√3 × √3)R30° superstructure are visible, as well as small Cu(111) areas and disordered patches. Oxidation at 680 °C leads to the formation of a well-ordered flat alumina film with two very similar oxide structures. One oxide structure has a nearly commensurate rectangular cell rotated by 30° with respect to a close-packed row of the substrate and grows in three different domains. The second structure has a commensurate cell consisting of four equivalent building blocks and has a rectangular centred symmetry. This structure is rotated by 18° with respect to a close-packed row of the substrate and grows in six different domains. The rectangular building blocks of these two oxide structures have a similar thickness, the same surface termination and the same number and arrangement of the atoms as the oxide film on NiAl(110) [G. Kresse et al., Science 308 (2005) 1448]. In contrast to the oxide on NiAl(110), alumina on the Cu-Al alloy crystal does not show stress-induced domain boundaries and grows in large defect-free domains. Thus, Pd deposited on this oxide nucleates not only on domain boundaries and steps, but also on the unperturbed oxide, forming (111)-oriented clusters.
Corresponding author: M. Schmid (schmid).
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