New magic numbers in metallic clusters: an unexpected metal dependence

Abstract

Using a many-body tight-binding potential within the second moment approximation in a quenched molecular dynamics simulation, we calculate the internal energy of free Cu, Ag and Au clusters of various sizes and morphologies. We find that the icosahedral structure, which is the equilibrium shape for small sizes at least for Cu and Ag, adopts a very inhomogeneous atomic relaxation. More surprisingly, introducing a vacancy at the center lowers the mean energy per atom for sufficiently large size icosahedra. This means that above a critical size, which decreases from Cu to Au, the icosahedron admits a constitutional vacancy. Taking into account the stability domain of the icosahedron relative to the fcc structure (namely the Wulff polyhedron), we find that there is a stability range of size for Cu and Ag icosahedra with a central vacancy, but not for Au icosahedra. This trend along the noble metal column is discussed in view of tight-binding potential parameters.