The surface of a disordered Pt25Co75(100) alloy has been investigated using quantitative LEED, AES and UHV-STM at room temperature. Atomic-resolution images reveal that it reconstructs with close-packed rows shifted by half the interatomic distance, from hollow to bridge sites. The density of shifted rows increases with the surface Pt concentration, leading to (1x5), (1x6) and (1x7) patterns. Segregation and chemical ordering lead to the formation of medium-range c(2x2) domains between the shifted rows. Chemical resolution was achieved with STM: the apparent height of the Pt atoms in the STM topographs is about 0.1-0.4 Å above that of Co, whereas LEED shows that Pt atoms are geometrically approx. 0.04 Å higher. The composition was determined down to the fourth layer. An oscillatory segregation profile is observed, with Pt-rich layers (<C1>=62.6% Pt, <C3>=53.5%), and Pt-depleted layers (<C2>=6.9%, <C4>=2.7%). Chemical ordering is present in the third layer and the four-layers surface slab stabilises with structure and a composition quite similar to that of the L12 PtCo3 Phase. As regards the composition and ordering of the top layer, there is a remarkable agreement between chemically resolved STM and LEED analysis.
Corresponding author: Y. Gauthier. Reprints also available from M. Schmid (schmid).
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