Surface alloying and dealloying in Bi/Cu(001) at low coverage

Using surface X-ray diffraction we have investigated the structure of the Bi/Cu surface alloy formed after Bi deposition on Cu(001) at room temperature. At low Bi coverage below about 0.35 ML (1 ML = 1.53 x 10(15) atoms cm(-2)), the intensity distribution along the integer order crystal truncation rods indicates surface alloying where Bi atoms substitute Cu surface atoms. The Bi atoms adsorb 0.61 (10) Angstrom above the surface Cu layer. At a coverage above about 0.35 ML where the formation of a well-ordered c(2 x 2) superstructure starts to set in, we observe surface dealloying indicated by a rapid decrease of the fractional Cu occupancy of the surface layer with increasing Bi coverage. The Bi atoms are located in hollow sites at an adsorption height of 2.27 (10) Angstrom corresponding to a hard sphere radius of 1.63 (8) Angstrom, somewhat below the metallic radius of 1.82 Angstrom. (C) 1997 Elsevier Science B.V.