NEW MAGIC NUMBERS IN METALLIC CLUSTERS - AN UNEXPECTED METAL DEPENDENCE

Using a many-body tight-binding potential within the second moment app roximation in a quenched molecular dynamics simulation, we calculate t he internal energy of free Cu, Ag and Au clusters of various sizes and morphologies. We find that the icosahedral structure, which is the eq uilibrium shape for small sizes at least for Cu and Ag adopts a very i nhomogeneous atomic relaxation, More surprisingly, introducing a vacan cy at the center lowers the mean energy per atom for sufficiently larg e size icosahedra. This means that above a critical size, which decrea ses from Cu to Au, the icosahedron admits a constitutional vacancy. Ta king into account the stability domain of the icosahedron relative to the fee structure (namely the Wulff polyhedron), we find that there is a stability range of size for Cu and Ag icosahedra with a central vac ancy, but not for Au icosahedra This trend along the noble metal colum n is discussed in view of tight-binding potential parameters. (C) 1997 Elsevier Science B.V.