SURFACE ALLOY IN THE C(4X4) PHASE OF PB ON CU(100)

The structure of the c(4 x 4) phase of Pb on Cu(100) has been determin ed by a LEED 1/V analysis. The characteristic feature of this structur e are Pb chains along the close packed rows with Cu chains between the Ph chains. The surface layer is an ordered alloy layer with 3 Pb and 4 Cu atoms in the primitive unit cell, the Pb coverage is 0.375 monola yers. The Pb-Pb distance within the chain is 3.4 Angstrom and 2.8% sma ller than the distances in bulk Pb. The Pb chain is slightly corrugate d by 0.05 Angstrom where one of the three Pb atoms is in a hollow site of the substrate. Due to the smaller atomic radius the Cu chain is 0. 65 Angstrom below the Pb site. Half of the Cu atoms are shifted by 0.2 3 Angstrom off the fourfold sites such that roughly the same distance to the surrounding Pb atoms in the top layer is maintained. The Pb-Cu bond length varies slightly at different sites around 2.9 Angstrom.