Abstract
Using the density functional theory method, the icosahedral TM@X12 (M@X12) clusters (TM=Mn, Tc, Re; M=Zn, Cd, Hg; and X=Sn, Ge), which are composed of Sn12 (Ge12) shell covering a single TM (M) atom, have been systematically examined to explore the role of TM’s (M’s) d valence electrons playing in the clusters. The results show that the magnetism originate from the contribution of TM’s d valence electrons to TM@X12 clusters, where TM’s (M’s) d valence electrons are not included in the superatomic electronic states to TM@X12 (M@X12) clusters. Taking into account the structural stability (imaginary frequency, binding energy, embedding energy, and core-shell interaction) as well as the chemical stability (HOMO-LUMO gap) after, we proposed that TM@X12 and M@X12 clusters can be assigned as the protyle superatoms. Furthermore, the results suggest that M@C60 clusters can not be superatoms, because their negative embedding energies and the distance from the center atom (M) to C atom is larger than the sum of their Van Waals radii. Interestingly enough, we may obtain a simple judging method: for a magnetic superatom, the smaller the energy gap between the highest occupied magnetic state (HOMS) and Fermi level or HOMO (MOgap, or MFgap), the easier on the change of its spin magnetic moment.
Highlights
The geometrical optimizations are performed through employing the DMOL simulation program package,[20] using the effective core potential (ECP),[21,22] double-numerical basis sets with p-polarization function (DNP), and PW91 exchange correlation potential in the generalized gradient approximation (GGA),[21,23] to gain the magnetic ground state of the superatomic characteristics of M@X12 (TM@X12), M@X12 and M@C60 clusters, which is more practical and effective than other software by turning on spin unrestricted of the DMOL simulation program package implanted in materials studio (MS)
Parallel analyses have been performed to M@X12, M@C60 and other TM@X12 clusters, the results show that the chemical stability of Tc@Sn12 (Tc@Ge12) cluster may be similar with that of single Tc atom, and the chemical stabilities of Re@Sn12, Tc@Ge12, M@X12 and M@C60 clusters significantly lower than that of Tc and M atoms, respectively
M-d valence state is exactly local and overlapping with total density of states (TDOS) at the lowest energy in DOS of M@X12 clusters shown in Fig. 5g–5l, which suggests that Magnetic analysis and roles of TM (M’s) d valence electrons are not able to offer any contribution to the chemical properties of M@X12 clusters
Summary
As a novel cluster owing to its superatomic electronic states like that of free atoms to case unusual physical and chemical properties, has attracted growing attentions in consideration of superatomic technological applications.[1,2,3] it is worth mentioning that, unlike free atom, the filling of superatomic orbitals (superorbitals) does not usually follow Hunds rule mainly due to their spans generally over multiple atoms and the degeneracy of electronic states is broken through Jahn-Teller effects.[1]Based on the extraordinary stability of superatoms, their optical, dielectric, magnetic, and catalytic properties have been studied in previous reports.[1,3,4] For instance, Zn@Ge12 and Cd@Sn12 clusters with a large HOMO-LUMO gap of about 2 eV may be used to assemble optoelectronic materials;[5] MnCan (n=6-15) clusters,[6] Mn@Sn12 cluster[4,7,8] MnSr9 cluster,[9] TcMg8 cluster[10] and V-alkali clusters (VLi8,11 VNa812 and VCs813) magnetic superatoms show a large spin magnetic moment of 5 μB, where the superior stability of Mn@Sn12 cluster[4,14] was confirmed in experiments. Superatoms have some superior chemical properties, such as the behaves of Al13 superatom in configuration of 1S21P61D102S21F142P5 like a halogen atom exhibit a high electron affinity,[15] where 2P5 state is offered via the s, p-valence electrons of Al atoms; besides, the cation of B2Li11 cluster had been found to have a very low ionization potential (3.40-3.73 eV), so B2Li11 cluster was regarded as a new type of binuclear superalkali atom;[16] the cationic superatom compounds (M-F)+ (M=OLi4, NLi5, CLi6, BLi7, and Al14) with low vertical electron affinities show that M can be seen a kind of superalkaline-earth atom.[17] the spherical coreshell structures with the closed-shell d10 configuration of metal atoms as the core of superatom, such as [Sn@Cu12@Sn20]12 in bulk A12Cu12Sn21 intermetallics[18] and A@B12@A20 (A=Sn, Pb; B=Zn, Cd) in cluster phase,[19] the d10 valence electrons of metal atoms (Md10) in the superatoms are not taken into account to fill the superatomic orbitals, or their endohedral icosahedra usually shares a closed-shell d10 configuration of metal atoms, which suggest that the Md10 is not contributed to superatomic chemical properties
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