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. 2013 May 24;2013:468327. doi: 10.1155/2013/468327

First-Principles Study on Stability and Magnetism of NimAln (m = 1–3, n = 1–9) Clusters

Xiao Zhang 1, Bao-Xing Li 1,*, Zhi-wei Ma 1, Jiao-jiao Gu 1
PMCID: PMC3677613  PMID: 23766691

Abstract

The investigation on the structures, stabilities, and magnetism of NimAln (m = 1–3, n = 1–9) clusters has been made by using first principles. We found some new ground-state structures which had not been found before. These mixed species prefer to adopt three-dimensional (3D) structures starting from four atoms. All the ground-state structures for the Ni-Al clusters are different from those of the corresponding pure Al clusters with the same number of atoms except for three atoms. The Mulliken population analysis shows that some charges transfer from the Al atoms to the Ni atoms. NiAln (n = odd number) cations, Ni2Al6 neutral, Ni2Al1 and Ni3Al cations and anions, and Ni3Al5 anion have the magnetic moments of 2 μ B. The magnetic moments of NiAl4 and NiAl6 cluster neutrals and cations are 2 μ B and 3 μ B, respectively. All the other cluster neutrals and ions do not have any nontrivial magnetic moments. The 3d electrons in Ni atoms are mainly responsible for the magnetism of the mixed Ni-Al clusters.

1. Introduction

Aluminum is one of the most popular metals used in quantum-effect electronic devices. It attracts scientists' great attention to its nanoscale clusters. Cohen et al. have explained the stability of small Aln clusters by using jellium model because of the valence electrons of Al-like free electron [1, 2]. Khanna et al. have discovered that aluminum clusters have chemical properties similar to single atoms of metallic and nonmetallic elements when they react with iodine [3, 4]. For instance, Al13 cluster behaves like a single iodine atom, while Al14 cluster is similar to an alkaline earth atom. Fournier have obtained the ground-state structures of Aln, Aln −,   and  Aln +  (4 ≤ n ≤ 15) by performing “Tabu Search” (TS) global optimizations directly on the BPW91/LANL2DZ potential energy surface [5]. They have found that all clusters (4 ≤ n ≤ 15) have the lowest spin state as their ground state except Al4 (triplet), Al4 + (quartet), Al7 − (triplet), and maybe Al5 + (singlet and triplet are degenerate). Furthermore, Al7 + and Al13 − had special stability and larger HOMO-LUMO gaps.

The studies of the Al clusters doped with impurity atoms have been also reported. Ren and Li have investigated the structures, stabilities, and magnetism of zinc-doped Aln (n = 1–9) clusters in detail by using first-principles density functional theory [6]. Ding and Li have done the calculation on AlnSim−n  (m = 6, 9,  10,  n ≤ m) by using the first-principles methods [7]. Li and Wang have performed first-principles calculations on the ground states of both neutral and anionic Al12 X  (X = C, Si, Ge, Sn, Pb) clusters [8].

Nickel is a transition metal. Aluminum clusters doped Ni atoms have many novel properties and chemical properties. NinAl  (n = 2–8) neutral clusters have been investigated by Wen et al. using the density functional theory and generalized gradient approximation (GGA) with the exchange-correlation potential (BPW91) [9]. It is found that the atomization energies per atom have the same trend as the binding energies per atom for Nin  (n = 3–9) clusters. It is shown that Ni5Al is the relatively most stable structure in the series. Besides that,  Ni average magnetic moment decreases when alloyed with Al atoms than that in pure Ni clusters. Deshpande et al. have reported the magnetic properties of small Ni13−nAln clusters with n = 0–13 calculated in the framework of density functional theory [10]. The overall magnetic moment of the Ni13−nAln cluster decreases with the sequential substitution of Ni by Al atoms. This could be attributed to the antiferromagnetic alignment of small individual atomic moments arising due to the hybridization of Al-p and Ni-3d orbitals. Calleja et al. have reported ab initio molecular dynamics simulations of Ni2,  Al2,  Ni13,  Al13,  and  Ni12Al clusters using a fully self-consistent density-functional method [11]. Recently, Sun et al. have performed the study on the growth behavior and electronic properties of NiAln  (n = 1–14) clusters in the framework of density-functional theory (DFT) theoretically [12]. In the ground-state structures of NiAln clusters, the equilibrium site of Ni atom gradually moves from convex and surface to interior site as the number of Al atom varying from 2 to 14. Ferrando et al. have performed very thorough review on alloyed clusters [13]. Bailey et al. have presented their research results about nickel and aluminum clusters and nickel-aluminum nanoalloy clusters with up to 55 atoms [14]. The effect of doping Al atoms into pure Ni clusters and vice versa has been investigated.

Although the investigations about Al, Ni, and NinAl clusters mentioned above have been performed, our knowledge about the properties of the Ni-Al clusters with different Ni/Al atomic ratio is still limited. In this paper, our main goal is to explore the effect of the Ni atoms on the geometric, electronic, and magnetic properties of the pure Aln  (n = 2–9) clusters.

The paper is organized as follows: a brief account of the computational methodology is given in Section 2, followed by a detailed presentation and discussion of the structures of different size NimAln  (n = 1–9, m = 1–3) clusters in Section 3. A summary of the findings and conclusions is given in Section 4.

2. Computational Method

We optimized the Ni-Al cluster structures by using first-principles density functional theory. Our calculations were performed with the generalized gradient approximation (GGA) by means of the Becke-Perdew functional, which uses Becke's [15] gradient correction to the local expression for the exchange energy and Perdew's [16] gradient correction to the local expression of the correlation energy, as implemented in the Amsterdam Density Functional (ADF) codes [17]. The choice of such a gradient-corrected exchange-energy functional gives us satisfactory results, which are in good agreement with the experimental results. For example, the calculated vertical electron affinity and vertical ionization potential for Al5 cluster are 2.16 eV and 6.61 eV, respectively. The available experimental values are 2.25 eV and 6.45 eV, respectively [18, 19]. In the ADF program, molecular orbitals (MOs) were expanded using a large, uncontracted set of Slater-type orbitals (STOs): TZ2P. The TZ2P basis is an all-electron basis of triple-ζ quality, augmented by two sets of polarization functions. The frozen-core approximation for the inner-core electrons was used. The orbitals up to 3p  for nickel and 2p for aluminum were kept frozen. An auxiliary set of s, p, d, f, and  g STOs was used to fit the molecular density and to represent the Coulomb and exchange potentials accurately in each self-consistent field (SCF) cycle. The self-consistent field was converged to a value of 10−5 in Hartree.

Frequency analyses at the stable structures are carried out at the same theoretical level to clarify if they are true minima or transition states on the potential energy surfaces of specific clusters. All of the most stable clusters in the paper are characterized as energy minima without imaginary frequencies.

3. Results and Discussions

Searching for the ground-state structures of the atomic clusters, especially the mixed clusters, is a difficult task. The following approach is applied in an attempt to find the global energy minimum structures. Firstly, about more than ten thousand initial geometrical configurations are produced by random selections of atomic positions within a three-dimensional box, or a cage, or a ball in real space. The distance between two nearest atoms is properly chosen to avoid overlapping or loosely packing. Next, the Amsterdam Density Functional (ADF) package is used for geometrical optimizations with their spins being not unrestricted. After the structural optimization on the initial configurations is performed, it is found that some structures are stable, but others are not convergent. Sometimes, several initial structures can gave out the same final structure after the first-principles optimizations. In the end, the structures with the largest binding energies calculated using generalized gradient approximation (GGA) are considered to be the ground-state geometries. The binding energy (BE) for the NimAln cluster is calculated according to the following atomization reaction: NimA1n → mNi + nA1. It is defined by the following: BE = mE Ni + nE A1 − E NimA1n, where E NimA1n is the total energy of the NimAln cluster. E Ni and E Al are the energy of Ni and Al atoms, respectively.

All the lowest energy structures of Al3, Al4, and Al5 clusters are planar structures. They are an equilateral triangle, a planar quadrangle, a trapezoid-like, respectively. The ground-state structures for Aln  (n = 6–10) clusters are three-dimensional. The Al6 and Al7 clusters have a distorted tetragonal bipyramid (D2d) and an antitrigonal prism with a capping atom (C3v), respectively. But, Fournier found that the ground-state structure for the Al6 cluster was a distorted trigonal prism by using the BPW91/LANL2DZ method [5]. For the Al7 cluster, its ground-state structure can be obtained by putting an atom on one triangular face of the prism for the Al6 cluster. We have also performed calculations on the two structures found by Fournier with the ADF program. It is found that both of them are metastable. Our results are 0.15 and 0.36 eV more stable than his, respectively. For the Al8 cluster, our result is a transcapped octahedron with C2 h symmetry. But, in Fournier's report, the ground-state structure is a distorted transcapped octahedron, or an antitrigonal prism with a face-capping atom and an edge-capping atom. We have performed structural optimization on his structure. It is found that his structure finally changes into the C2h structure. The Al9 cluster has a C2v ground-state structure. The most stable structure of pure Al10 cluster can be obtained from the Al9 ground-state isomer by putting a capping atom. All of the structures obtained by the ADF program are in excellent agreement with those reported by Chuang et al. using a genetic algorithm coupled with a tight-binding interatomic potential [20]. They are shown in Figure 1.

Figure 1.

Figure 1

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The ground-state structures of the Aln  (n = 3–10) clusters.

The ground-state structures of NiAln  (n = 2–9) clusters are presented in Figure 2. The ionic structures similar to the neutral structures are not repeated in Figure 2. But the structure of NiAl9 cluster cation is presented in Figure 2 because it has a severe distorted ground-state structure compared with its neutral and anion. Their symmetries are listed in Tables 1, 2, and 3. The NiAl3 and NiAl4 clusters and their ions have one three-dimensional structure, which are different from the planar structures of the Al4 and Al5 clusters. All the most stable structures for the NiAln  (n = 5–9) cluster neutrals, anions and cations are three-dimensional structures, but they are obviously different from the structures of the corresponding Aln  (n = 6–10) clusters with the same number of atoms. Positive NiAl9 cluster ion with C1 symmetry has a severe distorted ground-state structure compared with its neutral and anion (see the NiAl9 + structure in Figure 2). Mulliken population analysis shows that nonuniform charge loss of the Al atoms in the NiAl9 + structure results in its severe structure distortion.

Figure 2.

Figure 2

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The ground-state structures of the NiAln  (n = 2–9) clusters. The black ball refers to Ni atom in the clusters.

Table 1.

The symmetries, the total magnetic moments (M, in μ B), and the binding energies (BE, in eV) for NiAln (n = 2–9) clusters and their ions. The energy gaps (E g, in eV), the vertical electron affinities (EA, in eV), and vertical ionization potentials (IP, in eV) of neutral NiAln (n = 2–9) clusters.

Clusters Symmetries M (in μ B) BE (in eV) E g (in eV) EA (in eV) IP (in eV)
Anion Neutral Cation Anion Neutral Cation
NiAl C∞v 0 1 2 4.82 3.53 3.47 0.25 1.29 7.00
NiAl2 C2v 1 0 1 8.18 6.94 0.15 1.06 1.24 6.79
NiAl3 C3v 0 1 2 11.31 9.92 2.67 0.60 1.39 7.25
NiAl4 C4v 1 2 3 14.10 12.21 5.17 1.18 1.89 7.04
NiAl5 Cs 0 1 2 16.62 14.59 8.38 0.52 2.03 6.21
NiAl6 C2v 1 2 3 20.10 17.64 11.25 0.26 2.46 6.39
NiAl7 Cs 0 1 2 23.16 21.06 14.50 0.53 2.10 6.56
NiAl8 Cs 1 0 1 25.80 23.56 17.47 0.73 2.24 6.09
NiAl9 Cs 0 1 2 28.81 26.39 19.99 0.69 2.42 6.60
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Table 2.

The symmetries, the total magnetic moments (M, in μ B), and the binding energies (BE, in eV) for Ni2Aln (n = 2–9) clusters and their ions. The energy gaps (E g, in eV), the electron affinities (EA, in eV), and ionization potentials (IP, in eV) of neutral Ni2Aln (n = 2–9) clusters.

Clusters Symmetries M (in μ B) BE (in eV) E g (in eV) EA (in eV) IP (in eV)
Anion Neutral Cation Anion Neutral Cation
Ni2Al C2v 2 1 2 8.70 7.19 −0.32 0.51 1.51 6.87
Ni2Al2 C2v 1 0 1 12.79 10.90 4.14 0.66 1.89 6.76
Ni2Al3 C2v 0 1 0 15.87 14.48 8.60 0.54 1.39 5.88
Ni2Al4 Cs 1 0 1 18.41 16.84 10.76 0.86 1.57 6.08
Ni2Al5a C1 0 1 0 21.58 19.61 13.73 0.41 1.97 5.88
Ni2Al6 C2v 1 2 1 25.40 23.05 16.69 0.33 2.35 6.36
Ni2Al7 C2v 0 1 0 28.14 26.07 20.27 0.43 2.07 5.80
Ni2Al8 C1 1 0 1 30.65 28.50 22.50 0.65 2.15 6.00
Ni2Al9 C2v 0 1 0 33.74 31.37 25.39 0.44 2.37 5.98
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Table 3.

The symmetries, the total magnetic moments (M, in μ B), and the binding energies (BE, in eV) for Ni3Aln (n = 2–9) clusters and their ions. The energy gaps (E g, in eV) and the electron affinities (EA).

Clusters Symmetries M (in μ B) BE (in eV) E g (in eV) EA (in eV) IP (in eV)
Anion Neutral Cation Anion Neutral Cation
Ni3Al C2v 2 0 2 12.35 10.89 4.32 0.47 1.46 6.57
Ni3Al2 C2v 1 0 1 16.67 14.99 8.45 0.62 1.68 6.55
Ni3Al3 Cs 0 1 0 19.83 18.32 12.42 0.50 1.51 5.90
Ni3Al4 C2v 1 0 1 23.26 21.71 15.10 1.27 1.55 6.61
Ni3Al5 C2v 2 1 0 26.44 24.53 18.91 0.22 1.91 5.62
Ni3Al6 C2v 1 0 1 30.71 28.39 22.04 0.41 2.32 6.35
Ni3Al7 Cs 0 1 0 33.30 31.07 25.15 0.29 2.23 5.92
Ni3Al8a C1 1 0 1 35.62 33.43 27.71 0.48 2.19 5.72
Ni3Al9a C1 0 1 0 38.71 36.22 30.32 0.29 2.49 5.90
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The cluster's magnetism is usually described by its magnetic moment. We define magnetic moment (in Bohr magnetron unit μ B) as the difference between the electronic occupation number of spin-up and spin-down. We think the cluster is magnetic if its structure with nonzero spin is more stable than that with zero spin. The investigation on the magnetic moments for the Ni-Al clusters shows that anions, neutrals, and cations for the NiAl2 and NiAl8 clusters have the magnetic moments of 1 μ B, 0, 1 μ B, respectively. Negative, neutral, and positive NiAl4 and NiAl6 clusters have the magnetic moments of 1 μ B, 2 μ B, and 3 μ B, respectively. The NiAl,  NiAl3,  NiAl5,  NiAl7,  NiAl9 clusters with odd-aluminum atoms have the magnetic moments of 1 μ B. Their negative ions have no magnetic moment, but the positive ions show the magnetic moment of 2 μ B.

The most stable structures of Ni2Aln  (n = 1–9) clusters are presented in Figure 3. It is found from observing Figure 3 that the geometries are also different from those in Figures 1 and 2. Obviously, the second impurity Ni atom further changes the structures of Al host clusters. The lowest energy structure Ni2Al5a of Ni2Al5 cluster which is obtained by a random way has a C1 symmetry. The second most stable structure Ni2Al5b which produced by substitution from Al8 has a Cs symmetry and lies 0.07 eV above Ni2Al5a structure. All the ground state structures of the Ni2Al6, Ni2Al7,  and  Ni2Al9 clusters have the same C2v symmetry. The Ni2Al7 cluster can be obtained by substitution, by a random way, or by putting a capping atom on the Ni2Al6 cluster. The Ni2Al8 cluster has a low C1 symmetry.

Figure 3.

Figure 3

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The ground-state structures of the Ni2Aln  (n = 2–9) clusters. The black ball refers to Ni atoms in the clusters.

By performing calculation on the magnetic moments for the Ni2Aln  (n = 1–9) clusters, we obtain the following results. All the Ni2Aln  (n = odd-number) clusters have the total magnetic moment of 1.0 μ B due to one unpaired electron. This suggests that the two impurity Ni atoms do not change the magnetism of the Al clusters. On the other hand, the Ni2Al2, Ni2Al4, and Ni2Al8 clusters with even number of electrons have no magnetic moment because all the electrons are paired together in their respective molecular orbitals. But, unexpectedly, the Ni2Al6 cluster shows magnetic moment of 2 μ B. Each Ni atom in the Ni2Al6 cluster has the local magnetic moment 0.40 μ B and the total charge transferring from the Al atoms to them is −0.74 e. However, the average magnetic moment of each Al atom is 0.20 μ B, which is only half of that of the Ni atoms. The Mulliken population analyses indicate that the local magnetic moment 0.40 μ B of Ni atoms is mainly from the d orbital. On the other hand, if we add up the magnetic moments for the Al atoms and the Ni atoms, respectively, the total magnetic moments for Al atoms are 1.2 μ B, which is larger than 0.8 μ B for Ni atoms. Hence, Al atoms contribute more to total spin than Ni atoms.

The Ni2Al−  and  Ni2Al+ cluster ions are expected to produce zero magnetic moment. But our calculations show that the ions have the magnetic moment of 2 μ B. It is found that the Ni2Al3,  Ni2Al5,  Ni2Al7, and  Ni2Al9 clusters ions have no magnetic moment. The magnetic moment for the Ni2Al4,  Ni2Al6,  and  Ni2Al8 cluster ions is 1 μ B. Furthermore, it is found that the electron added or removed in the neutral Ni2Aln  (n = 1–9) clusters does not change the basic geometrical structures.

The lowest energy structures of Ni3Aln  (n = 1–9) clusters are presented in Figure 4. All the structures are three-dimensional. They differ from the geometries of the NiAln and Ni2Aln  (n = 1–9) clusters in Figures 2 and 3. The clusters have C3v, C2v, Cs, or  C1 symmetry. The ground-state structure of the Ni3Al8 cluster is degenerate. The Ni3Al8a structure lies only 0.03 eV below the Ni3Al8b structure. The Ni3Al8b structure can be obtained by putting two Al capping atoms in the Ni3Al6 structure. The Ni3Al9a structure is 0.26 eV more stable than the Ni3Al9b structure.

Figure 4.

Figure 4

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The ground-state structures of the Ni3Aln  (n = 2–9) clusters. The black ball refers to Ni atoms in the clusters.

The Ni3Aln  (n = odd-number) clusters have the magnetic moment of 1.0 μ B due to one unpaired single electron. The magnetic moment is trivial. For the Ni3Aln  (n = even-number) clusters show no magnetic moment because the molecular orbitals are doubly occupied. For their ions, some clusters show magnetism. The Ni3Al−, Ni3Al+, and  Ni3Al5 − cluster ions are expected to produce zero magnetic moment. But our calculations indicate that they have the magnetic moment of 2 μ B. All the Ni3Al3, Ni3Al7, and Ni3Al9 clusters ions including the Ni3Al5 + cation have no magnetic moments. The magnetic moment for the Ni3Al4, Ni3Al6, and Ni3Al8 clusters ions is 1 μ B, which is trivial.

To investigate the bonding character of the mixed Ni-Al clusters in relation to molecular orbitals, we have also examined the density of states in terms of the contributions of the different orbital components (s, p, d) and the frontier orbital (HOMO and LUMO) states. The Mulliken population analyses indicate that some of charge transfer from the Al atoms to the Ni atoms. Here, we take the Ni2Al6 cluster with the magnetic moment of 2 μ B as an example to illustrate their bonding properties. The orbitals of HOMO and LUMO states of the Ni2Al6 cluster are shown in Figure 5. For the orbitals of the HOMO state, the cluster binds in the term of σ and π orbital. Likewise, the binging characteristic varies from σ orbital to π orbital and δ feature between Ni and Al atoms for the LUMO state.

Figure 5.

Figure 5

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The HOMO and LUMO orbitals of the Ni2Al6 cluster.

In order to better understand the relative stability of the Ni-Al clusters, we have also investigated the second difference of cluster energies, the energy gaps, and their positive and negative ion clusters. The second difference of cluster energies Δ2 E = E(NimAln+1) + E(NimAln−1) − 2E(NimAln)  (m = 1–3, n = 2–9), which is a sensitive quantity that reflects the relative stability of the mixed clusters. Maximum Δ2 E indicates that the cluster is more stable than its neighboring clusters. The maximums for the Ni1Aln  (n = 1–9)  and  Ni2Aln  (n = 1–9) clusters are found at n = 3  and  7 (see Figure 6). The conclusion is a good agreement with that obtained by Sun et al. [12]. For the Ni3Aln  (n = 1–9) clusters, the maximums are at n = 4  and  6. It is found from observing Figure 6 that the minimum for the mixed clusters is at n = 5. This implies that the NimAl5  (m = 1–3) clusters are less stable than their neighboring clusters.

Figure 6.

Figure 6

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The second difference of the cluster energies, the energy gaps E g between the highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO), the electron affinities (EA), and the ionization potentials (IP) for NimAln  (m = 1–3, n = 1–9) clusters.

The maximums of Δ2E for the pure Aln  (n = 2–10) clusters are also at n = 3  and  7. The stability probably can be explained by jellium model [2, 21]. In this model, the nuclei together with the innermost electrons form a positive charged background, whereupon the valence electrons coming from individual atoms are then subjected to this potential. The clusters containing 8, 20, 40, 58… valence electrons correspond to filled electronic shells and exhibit enhanced stability. But the stability is associated not only with closed electronic shells (Ne = 20, 40) but also, to a lesser extent, with numbers of electrons just above and below the shell closing numbers [5]. The simultaneous stability of the neutral, anion, and cation of a given size can happen only for clusters of elements having more than one itinerant electron per atom. Therefore, the higher stability of Al3  and  Al7 clusters is easily understood. It is found from observing Figure 6 that the mixed NimAl3  and  NimAl7  (m = 1  or  2) clusters also have high stability. This indicates that the stability of the nickel-doped Aln clusters is determined by the host Al clusters. But it should be noted that more heteroatoms would change their relative stability, as we have seen in the Ni3Aln  (n = 2–9) clusters. Al5 cluster is less stable compared to other Al clusters. This is the reason that the NimAl5  (m = 1–3) clusters are less stable than their neighboring clusters.

The energy gaps E gs as an important function of the cluster size present a characteristic quantity of metal clusters' chemical activity. Here we define E gs as the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). For the pure Aln clusters, it is found that the Al2 cluster has a large gap of 3.05 eV. Other gaps change within range from 0.32 to 0.84 eV. For the smaller mixed clusters, their energy gaps change drastically. But they show relatively small fluctuation as the atomic number increases. It is worth noting that the E g curves display an even/odd alternating pattern as a function of cluster size for the Ni3Aln  (n = 1–9) clusters. Even though the Ni1Aln  and  Ni2Aln  (n = 1–9) clusters with larger than five atoms do not present the even/odd alternating pattern, their change trend is similar as the atomic number increases.

Finally, we have also investigated the NimAln  (m = 1–3, n = 1–9) positive and negative ion clusters. The electron affinities EA = E(NimAln) − E(NimAln −)  (m = 1–3, n = 1–9) are the amount of energy released when the cluster obtains an electron from its neutral state. In our research, all the vertical electron affinities (EA) of the NimAln  (m = 1–3, n = 1–9) clusters are positive, suggesting that the clusters have a tendency to gain an electron, as it is energetically favorable to do so. The vertical electron affinities (EA) change with atomic number, shown as Figure 6. The graph of vertical electron affinities versus atomic number shows that the mixed clusters with one, two, and three nickel atoms have a similar change trend. For the positive ion clusters, similar energies called vertical ionization potentials are calculated. The graph of vertical ionization potentials versus atomic number displays different changes. For the Ni2Aln  (n = 2–9) clusters with two nickel atoms, the vertical ionization potentials (IP) show an even/odd alteration with the number of aluminum atoms. For the Ni3Aln  (n = 2–9) clusters with three nickel atoms, a similar change trend (except  for  n = 8) is observed. But, for the mixed clusters including one nickel atom, the property does not exist.

4. Conclusions

We have optimized the geometric structures of the mixed NimAln  (m = 1–3, n = 1–9) clusters by using first-principles density functional theory. Their ground-state structures are obtained. Most of them are different from those of the host Al clusters. The impurity Ni atoms result in the local structural distortion of the mixed clusters in comparison with the corresponding pure Al clusters. At the same time, we calculated magnetic moment of the mixed cluster neutrals and ions. Some of them present nontrivial magnetic moments. The NiAln  (n = odd  number) positive ions, the Ni2Al6  neutral, the  Ni2Al1  and  Ni3Al positive and negative ions, and the Ni3Al5 negative ion possess the magnetic moments of 2 μ B. NiAl4  and  NiAl6 cluster neutrals and positive ions have the magnetic moments of 2 μ B and 3 μ B, respectively. But all the other cluster neutrals and ions do not have any nontrivial magnetic moments. Some of charge transfer from the Al atoms to the Ni atoms. The local magnetic moment of Ni atoms is mainly from the d orbital. Both the structural effect and chemical bonding are responsible for the magnetism of the mixed Ni-Al clusters.

Acknowledgments

The Foundation for the Author of National Excellent Doctoral Dissertation of PR China (Grant No. 200320), the Natural Science Foundation of Zhejiang Province (Grant no. Y6100098), and the National Natural Science Foundation of China (Grant no. 11074062) supported this work.

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