Abstract
The stoichiometries and geometric structures formed by the segregation of C, Al and S on a Fe(100) surface have been investigated by Auger electron spectroscopy and quantitative low-energy electron diffraction (LEED). Step-wise annealing of a sputtered surface with increasing annealing temperature reveals the successive segregation of C, Al and S. According to quantitative LEED analyses, each segregand forms a distinct c(2 × 2) long-range ordered structure. Also, each segregand removes the preceding one from the surface entirely, i.e. segregation in the Fe(100)–(C, Al, S) system is purely competitive with no ordered co-segregation regimes involving two or even three elements. The c(2 × 2) phases of segregated carbon and sulfur consist of elemental surface adlayers with the adatoms residing in four-fold symmetric hollow sites of the iron substrate. This is in contrast to segregated Al which, according to an earlier analysis, forms a c(2 × 2)-symmetric surface alloy layer with iron. In all cases there is some chemical disorder within subsurface layers with Fe atoms substituted by Al. The bond lengths between the segregated adatoms and iron neighbours are close to the sum of the covalent radii of the elements involved whereby carbon appears to be five-fold coordinated, in contrast to four-fold coordinated sulfur.
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