The structures of the Cu(111) [GRAPHICS] and the Cu(111)(root 7 x root
7)R19.1 degrees-S reconstructions have been studied by surface X-ray
diffraction and two new structural models are proposed. Although the u
nit cells for the two structures are quite different, a high degree of
similarity has been revealed. It consists of a Cu4S unit with sulfur
adsorbed in the fourfold hollow site on top of a Cu tetramer. The \4-1
41--- structure contains two Cu4S units, the (root 7 x root 7)R19.1 de
grees structure only one. In both structures additional sulfur atoms a
re present. The sulfur coverages for the structures are 0.35 and 0.43
ML, respectively. The building block for the structures is similar to
the Ni4S unit which we recently found in the Ni(111)(5 root 3 x 2)-S s
tructure. We propose that the driving force for the reconstruction is
the chemical stability of the Cu4S units given by the tendency of sulf
ur to be fourfold coordinated. (C) 1997 Elsevier Science B.V.