Quantum effects on the structure of pure and binary metallic nanoclusters

A family of high-symmetry bimetallic clusters--recently shown to give rise to "magic" structures in the case of Ag-Cu and Ag-Ni nanoclusters--is investigated also in the case of Ag-Pd, Ag-Co, Au-Cu, Au-Ni, and Au-Co. Cluster structures obtained by global optimization within a semiempirical potential model are then reoptimized via density functional calculations. Sizes up to 45 atoms are considered. Ag-Cu, Ag-Ni, and Au-Ni clusters have some common characteristics. They present polyicosahedral character and achieve maximum stability at the Ag- and Au-rich compositions, when the structural arrangement is associated to a Ni?Cu?core- Ag?Au?shell chemical ordering. This is due both to the huge size mismatch between the components and a clear tendency of the larger atoms to segregate at the surface. In Au-Cu and Ag-Pd, clusters achieve their best stability at intermediate compositions, in agreement with the tendency of these metals to mix in the bulk phase. Finally, for Ag-Co and Au-Co, peculiar quantum effects favor intermediate compositions despite the fact that these metal phases separate in the bulk. These results are rationalized in terms of the interplay between electronic and volumetric effects on the structure of metallic nanoclusters.