Surface alloys, overlayer and incommensurate structures of Bi on Cu(1 1 1)

Abstract

Using surface X-ray diffraction, we have determined the structure of three different sub-monolayer phases of Bi on Cu(1 1 1). In contrast to an early report, we find that at a coverage of 1/3 monolayer a substitutional surface alloy is formed with a (√3 × √3) R30° 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(1 1 1) surface is the main driving force for the different phases.