FORMATION OF WELL-ORDERED SURFACE-COMPOUNDS BY COADSORPTION OF THALLIUM AND BROMIDE ON THE AU(111) ELECTRODE SURFACE

The structures of the adlayers formed on Au(111) in acid solution cont aining thallium and bromide have been investigated by using in situ su rface X-ray diffraction. At the most positive potentials, where Br for ms close-packed rotated-hexagonal monolayers, the surface is free of T l. The Br adlayer disorders at the potential where thallium starts to coadsorb. With further decreasing potential, two well-ordered commensu rate and one weakly-ordered incommensurate phases appear over a 0.7 V potential region prior to the formation of the close-packed rotated-he xagonal Tl monolayer, The structure factor analysis for the commensura te phases shows the formation of TlBr2 and TlBr surface compounds in t he 3-(root 13 x root 13) and 2-(3 x root 5) phases, respectively. In a ddition, an adlayer-induced subsurface relaxation has been observed. U nder the 2TlBr-(3 x root 3) adlayer, where bromide ions are at the bri dge sites and thallium ions are situated near the hollow sites, the to p layer of Au atoms displaces laterally along the root 3 direction. Th is results in more uniform spacings between T1 and the three neighbori ng gold atoms, as well as an increase of the Br-Au separation. (C) 199 8 Elsevier Science B.V. All rights reserved.