Magnetism of electrons in atoms and superatoms

Abstract

The quantum states of electrons in small symmetric metallic clusters are grouped into shells similar to the electronic shells in free atoms, leading to the conceptual basis for defining superatoms. The filling of the electronic shells in clusters, however, do not follow Hund's rule and usually result in non-magnetic species. It is shown that by embedding a transition metal in group II atoms, one can stabilize superatoms with unpaired electronic supershells. We demonstrate this intriguing effect through electronic structure studies of MnSr n (n ¼ 6-12) clusters within first principles generalized gradient calculations. The studies identify an unusually stable magnetic MnSr 9 species with a large exchange splitting of 1.82 eV of the superatomic D-states. It is shown that the exchange split d-states in the Mn atom induce exchange splitting in S and D superatomic shells because of the hybridization between orbitals of selected parity. The magnetic MnSr 9 cluster with 25 valence electrons has filled 1S 2 , 1P 6 , 1D 10 , 2S 2 shells, making it highly stable, and an open shell of 5 unpaired D electrons breeding the magnetic moment. The stable cluster is resistant to collapse as two motifs are united to form a supermolecule. V

The role of TM’s (M’s) d valence electrons in TM@X12 and M@X12 clusters

“…To achieve magnetic superatoms, a singely magnetic TM atom were embedded into a small simple-metal cluster, because either the D-state (magnetic state) of superatoms almost exclusively comes from atomic d-state (that is, for the filling superatomic shells, atomic d electrons are thought as delocalized valence electrons) or its superatomic D-state (magnetic state) hybridize strongly with atomic d-states (that is, atomic d electrons are strongly localized and not fill superatomic shells) in previous reports. 6,9,10,12,13,[29][30][31][32][33][34][35] The exchange-splitting between the majority of superatomic D shells (atomic d shells) and their minority states can resulted in by both cases of above, so a superatom can have a corresponding spin magnetic moment. However, the spin magnetic moment of one superatom may be very vulnerable to its surrounding environments, which restricts the practical applications of superatoms.…”

“…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@Ge 12 and Cd@Sn 12 clusters with a large HOMO-LUMO gap of about 2 eV may be used to assemble optoelectronic materials; 5 accordingly, MnCa n (n=6-15) clusters, 6 Mn@Sn 12 cluster 4,7,8 MnSr 9 cluster, 9 TcMg 8 cluster 10 and V-alkali clusters (VLi 8 , 11 VNa 8 12 and VCs 8 13 ) magnetic superatoms show a large spin magnetic moment of 5 µ B , where the superior stability of Mn@Sn 12 cluster 4,14 was confirmed in experiments. So far, it is generally accepted that the spin magnetic moment of magnetic superatoms is offered by atomic d-state electrons (or superatomic D states) localized on TM sites, while its superatomic stability is provided via the delocalized s, p-valence electrons of metal atoms fully occupied diffuse superatomic S, P states.…”

Design of Three-shell Icosahedral Matryoshka Clusters A@B12@A20 (A = Sn, Pb; B = Mg, Zn, Cd, Mn)

“…The concept of magnetic superatom was initially introduced by Kumar and Kawazoe 30 , who predicted Mn@X 12 clusters (X = Ge, Sn) as icosahedral magnetic superatoms with a high magnetic moment of 5 μ B and a large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap of ~1.1 eV. By appropriately combining the localized d states with magnetism and the delocalized superatom orbitals that stabilize the entire cluster, Khanna and co-workers recently designed a series of magnetic superatoms 31 , such as VCs 8 32 , VNa 8 − 33 , Mg 8 Fe 34 , Ca 8 Fe 35 , MnCa 9 36 , MnSr 9 37 , ScK 12 and ScCs 12 38 . All of them possess enhanced stabilities, substantial HOMO-LUMO gaps (usually about 0.4 ~ 0.7 eV), and large magnetic moments on the transition metal atom (up to 5 μ B ).…”

The magnetic binary lithium clusters W2Lin (n = 15‐19): A theoretical prediction of “di‐superatomic molecules”

“…[26,27] The variation of E gap in the W 2 @Li n clusters shows that the lowest-lying isomer of W 2 @Li 15 has the largest gap of 0.49 eV (PW91/SDD) in the series, corresponding to its high stability. The gaps of other isomers are moderate in comparison with that of the typical magnetic superatoms, such as V@Na 8 (0.69 eV), [5] MnSr 9 (0.35 eV), [28] and VNa 8 − (0.42 eV). [6] To evaluate the feasibility of the two W atoms encapsulated in the Li cage, the interaction of the embedded atoms and the outer cage, namely, the embedding energy (D e ), is calculated using the following equation…”