Structural study of the commensurate-incommensurate low-temperature phase transition of Pb on Si (111)

A close-packed lead adsorbate layer on Si(111) forms an incommensurate hexagonal phase which exhibits a reversible phase transition to a commensurate Si(111)-((3)(2)(-1)(1))-Pb structure upon cooling below 230 K. Surface X-ray diffraction measurements with synchrotron radiation reveal that the structure consists of a minimally distorted hexagonal lead overlayer with a coverage of nominally 1.2 ML on top of an essentially undisturbed silicon substrate. The interatomic distances indicate that all dangling bonds of the substrate are saturated. The stability of the commensurate phase can be explained by the presence of bonds with covalent character between the adsorbate atoms in on-top sites and the substrate atoms and within the otherwise metallic overlayer, (C) 2000 Elsevier Science B.V. All rights reserved.