The Stability And Magnetic Moments Of Small Ni-H Clusters: A Study Based On All-electron DFT

Clusters are aggregates of atoms or molecules ranging in size from two to tens of thounsands of monomer units. The size limit to what we may call a cluster has something to do with the onset of bulk behavior. The study of clusters can not only help to making more understanding of native properties of atomic clusters, but also speed learning of properties of solid body. Structural stability, electronic structure, and magnetic properties are three important linked problems in cluster research, since the magnetic moment depends on the electronic structure, which in turn depends on the geometry. Knowledge concerning these questions can give us additional insight into the physical and chemical properties.Ni-H cluster had been extensively investigated because the process of H adsorbed on Ni, Pd, and other transition metal surface is the major steps of catalysis, corrosion, hydrogen-storage, and other chemical reactions. In this paper, using all-electron DFT-GGA calculation, the lowest-energy structures of Ni_nH (n≤13) and Ni_nH₂ (n≤9) clusters are obtained by considering a large number of structural isomers for each cluster size.The binding manner of hydrogen and nickel clusters evolves with the clusters size. In the Ni_nH(n≤13) clusters, the atomic H tends to bond at bridge sites for smaller clusters with n≤8. Starting from n=9 to 13, the H atom prefers to occupy the surface site. The magnetic moments of nickel clusters are decreased by bond atomic hydrogen except n=3,4. Furthermore, the second derivative of the binding energy has also been discussed. Local peak is found at n=10, implying that these clusters possess relatively higher stability than their neighbors. The results indicate that the doped H atom enhances the stability of nickel clusters and the Ni10H cluster is the most stable structure.For the di-hydrogenated nickel clusters, we find that the H₂ molecule is dissociated on the nickel cluster surface .The preferred arrangement of the two hydrogen atoms on the surface is in two neighboring sites, so that one nickel atom coordinates to both hydrogens. The magnetic moments of Ni_nH₂(n≤9) show an oscillatory behavior. We have also calculated the second derivative of the binding energy, which indicates that Ni₇H₂ is a magic-number cluster.