Chemical ordening and reconstruction of Pt25Co75(100): a LEED/STM study

Y. Gauthier, P. Dolle, R. Baudoing-Savois, W. Hebenstreit*, E. Platzgummer*, M. Schmid*, P. Varga*

Laboratoire de Cristallographie, CNRS, BP 166 X, 38042 Grenoble Cedex 09, France
*Institut für Allgemeine Physik, Technische Universität Wien, A-1040 Wien, Austria

Surf. Sci. 396 (1998) 137-155

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< encoded email address >).

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Part of this work is on display in the IAP/TU Wien STM Gallery (see the chemical contrast page).