Stable structural and magnetic isomers of small transition-metal clusters from the Ni group:: an ab initio density-functional study

We present ab initio density-functional studies of structural and magnetic isomers of Ni-N and Pt-N clusters with up to 13 atoms. Our investigations are based on fixed-moment calculations within a spin-polarized generalized gradient approximation and on static as well as dynamical optimizations of the cluster-structure, using quantum-mechanical many-body forces calculated via the Hellmann-Feynman theorem. Together with our earlier paper on Pd-N clusters (Futschek et al 2005 J. Phys.: Condens. Matter 17 5927-63) the present work completes a comprehensive investigation of small clusters formed by metals of the Pt group of the periodic table. We discuss the trends in structure, binding energy and magnetic moments as a function of cluster size and through the 3d-4d-5d series. We demonstrate that the transition from the more localized 3d to the more extended 5d orbitals influences not only the magnetic ground state, but also the geometric structure of the clusters. The difference is most pronounced for the largest clusters in this series (N = 11, 12, 13) where the Ni clusters adopt a polytetrahedral arrangement converging to the Ni-13 icosahedron, whereas the structures of Pd clusters and Pt clusters are based on octahedral motifs closely resembling fragments of the face-centred cubic structure of the bulk metals.