Theoretical Studies On The Structures And Properties Of Nickel Clusters

Clusters, as a transition state from molecular to macroscopic materials, have attracted much attention and interest in both theoretical and experimental studies. The structural and property changes of clusters with cluster size can reflect the micro to macro transformation at some extent. In recent years, transition metal clusters are known to play an important role in homogeneous and heterogeneous catalysis, laser physics, crystal and thin film growth, organometallic chemistry, and nanoscale electronic materials and devices. Among the 30 kinds of pure transition metal clusters, nickel clusters are the primary target of many research groups because of their extensive catalytic and important magnetic properties.In this work, we use the genetic algorithm (GA) coupled with a TB potential of Ni to search for low-energy candidates of Ni clusters. The low-energy candidate structures from the GA/TB search were further optimized by using the density functional theory calculations with the PBE exchange-correlation energy functional. We search for stable structures of medium-sized Ni_n (n = 20-35) clusters, and the binding energies, second differences in energies, magnetic properties, the adiabatic electronic affinities, adiabatic ionization potentials have been caculated in order to analyze the stabilities and electronic structures of the clusters. The main research results are listed as follows:(1): The lowest-energy structures of the neutral nickel clusters Ni_n (n = 20-35) were studied by a combination method of the genetic algorithm searching with a tight-binding potential and the density functional theory calculations. The DFT-PBE calculations were performed using the VASP code. The medium-sized nickel clusters in this size range from 20 to 35 atoms are found to favor the double-icosahedron-like structures. For example, Ni₂₀ can be viewed as adding one atom to the side face in the waist of Ni₁₉; Ni₂₁ by adding two atoms on the adjacent waist sites of Ni₁₉; Ni₂₃ by adding a Ni₄ segment on Ni₁₉, etc. Ni₂₃ and Ni₂₆ can be seen as three interpenetrating 13-atom icosahedrons and two interpenetrating 19-atom double-icosahedrons, respectively. The structure of Ni₂₉ is considered to be composed of two interpenetrating double-icosahedron and double-icositetrahedron (with three parallel hexagonal rings).(2) We have carried out a systematic study on relative stabilities of nickel clusters Ni_n (n = 2-30), including binding energies and second differences in energies. The binding energy curve can be roughly divided into four regions: the binding energy increase rapidly for n < 6; moderately for 6 < n < 10; smoothly for Ni₁₁ and Ni₁₂; and slowly for n > 13. The slight humps on the curve indicate higher stability for some specific clusters. The curve of the second differences in energies shows an oscillating behavior for Ni_n at n = 2, 6, 10, 11, 13, 20, 23, 26, and 29, suggesting that these clusters are more stable compared to their neighbors. For ionic clusters, there are several peaks on the binding energy curve at n =3, 4, 11, 13, 23, 26 and 28 for the cationic clusters, and at n = 6, 9, 11, 16, 18, 23, 26 and 29 for the anionic clusters, showing these cluster ions are relatively more stable. (3) The calculated magnetic moments of Ni_n (n = 1-30) reveal that Ni3, 6,13,16,19,23 have relatively smaller magnetic moments both from the experimental and theoretical results. The magnetic moments of the cationic and anionic nickel clusters have been shown that Ni_{4, 13, 15, 19, 21, 24}⁺ and Ni_{4, 13, 19, 23}^- have relatively smaller magnetic moments compared with their neighbors. All the Ni_13,19 neutral and ionic clusters have smaller magnetic moments. We can consider that the icosahedron and double icosahedron structures of Ni13,19 have special characters which can lower their magnetic moment of the whole system.(4) The adiabatic ionization potentials (AIPs) and the adiabatic electron affinities (AEAs) of the Ni_n ( n-30) clusters have been calculated based on the lowest-energy structures of the neutral and ionic clusters of the Ni_n (n-30). The behaviors of the calculated ionization potentials for the Ni_n (n-30) clusters agree well with the experimental results, though the calculated AIPs are smaller than the experiment data. It can be seen from the plot that there is an oscillating behavior at the size range of 3-10. In particular, there correspond to the sharp rises at Ni6 and Ni8 on the AIP curve. For the Ni_n (n = 6-30), the AIP decreases slowly as the cluster size increases. The tendency of the calculated electron affinity is generally consistent but has small deviations compared with the experimental result. Ni2 has the lowest AEA value. Moreover, Ni_10,16,19 shows slightly smaller AEAs than their neighbors.