Surface alloys, overlayer and incommensurate structures of Bi on Cu(111)

Using surface X-ray diffraction, we have determined the structure of three different sub-monolayer phases of Bi on Cu(111). In contrast to an early report, we find that at a coverage of 1/3 monolayer a substitutional surface alloy is formed with a (root3 x root3)R30degrees unit cell. For increasing coverage, de-alloying occurs, leading to an overlayer structure at a coverage of 0.5 ML in which the Bi atoms form zigzag chains. The surface contains three domains of this ((2 0)(1 2))phase. Finally, at a slightly higher coverage of 0.53 ML, the ((2 0) (1 2))unit cell is compressed in one direction, leading to a uniaxial-incommensurate phase with three rotational domains. The structure determination includes relaxations in the topmost layers and therefore allows a detailed comparison of the most important bond distances. This shows that an increased charge density of the Cu(111) surface is the main driving force for the different phases. (C) 2004 Elsevier B.V. All rights reserved.