Ni clusters

Figure Cl. 1.3 shows a plot of tire chemical reactivity of small Fe, Co and Ni clusters witli FI2 as a function of size (full curves) [53]. The reactivity changes by several orders of magnitudes simply by changing tire cluster size by one atom. Botli geometrical and electronic arguments have been put fortli to explain such reactivity changes. It is found tliat tire reactivity correlates witli tire difference between tire ionization potential (IP) and tire electron affinity...

By CNDO calculation BE is an increasing function of cluster size for Ag clusters and for Ni clusters [54]. The calculations for Ni clusters showed that the contribution an atom makes to the total BE is proportional to its coordination number [54]. The orbital energies of Ni follow a smooth function of cluster size. As size increases, LUMO decreases and HOMO increases. This represents a convergence of IP and electron affinity values with increase in size. 

Fig. 32. Hydrogen (empty circle) adsorption on Ni clusters, (1) (111) surface (2) (100) surface (5) (110) surface (4) (111) surface hole (5) (100) surface hole.

Table III. Results of non-iterative calculations for Ni clusters (Ref. 46). Numbering of atoms as in Fig. 31.

The magnetic moments of Fe, Co, and Ni clusters with sizes up to 700 atoms have been measured by Billas et al.1,2 Those measurements were made under conditions where the clusters exhibit super-paramagnetic behavior for low cluster temperatures (vibrational temperature Tv b = 78 K for Ni and Co clusters and 120 K for Fe clusters). Their results are shown in Figure 1. As... 

Figure 2 Comparison between the experimental average magnetic moments of Ni clusters measured by Apsel et al.3 (black dots) and the moments calculated by a tight binding method45,4 (light circles). Reproduced with permission from Ref. 44.

Figure 4 Ground state geometries of Ni clusters with 5 to 16 atoms (symmetries are indicated), obtained using the Gupta potential. Reproduced with permission from Ref. 45.

An alternative model for explaining the behavior of the magnetic moment of Ni clusters has been proposed by Fujima and Yamaguchi (FY).56 The interest of this model is because it may contain some additional ingredients required to explain the observed maxima of jl. However, as a function of N, the FY model cannot predict the minima. It is intriguing that the observed maxima of ft are at N = 8 and near N = 20 and N = 40.3 These numbers... 

Wan and coworkers48 studied the magnetic moments of Ni clusters, taking into account both spin and orbital effects. Wan et al.48 used the following TB Hamiltonian... 

The spin magnetic moments of Ni clusters calculated by Wan et al.48 are in reasonable agreement with density functional calculations,61,63,72 but both approaches, that is, TB and DFT, give values substantially smaller than the experimental magnetic moments. The results of Wan et al. improve by adding... 

Figure 8 Calculated spin, orbital, and total magnetic moments per atom of Ni clusters. Reproduced with permission from Ref. 48.

The magnetic moments of the Ni clusters are dominated by the contribution from surface atoms.48,69 The analysis of Wan et al. indicates that the orbital and spin local moments of cluster atoms with atomic coordination 8 or larger are similar to those in the bulk (p spin 0.55 and orb 0.05 pB) 73 that is, the orbital moment is almost quenched for internal cluster atoms. In contrast, there is a large enhancement of the spin and orbital moments for atoms with coordination less than 8. This enhancement increases with the coordination deficit, and it is larger for the orbital moment. Wan et al.48 also analyzed the quantum confinement effect proposed by Fujima and Yamaguchi,56 i.e., the... 

Theoretical Study of Icosahedral Ni Clusters within the Embedded Atom Method. 

Tight Binding Molecular Dynamics Study of Ni Clusters. 

Ni Clusters. Two new nickel compounds, [Nin Nin(MeOH)3 8(p.-CN)3o Mv(CN)3 6], where Mv is either Mov or Wv, have been studied54 by HFEPR at 230 GHz. Both compounds form neutral cluster molecules, corresponding to NiM-centred polyhedrons spanned by 14 peripheral metal ions. 

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