A DFT Investigation On Eu, Mn Encapsulated Alkali-metal Magnetic Superatoms

Atomic clusters with suitable size and composition can be designed to mimic the chemistry of atoms in the Periodic Table. These clusters which can be viewed as "super atoms" could then form the building blocks for a class of solids with unique structural, electronic, optical, magnetic, and thermodynamic properties,inducing the upsurge of studies. It is reported the magnetic and electrical superatoms VCs8, which may have potential applications in the field of molecular electronics, and people pay more attention on atomic surperatom clusters.In this paper, all the calculations are performed using density functional theory. Based on Morse potential, one hundred thirty-five kinds of Eu @A_n (A = Li, Na, K, Rb, Cs; n = 15-18) cage-like clusters, which europium atoms embedded in this cage, was built and their stability are predicted. The electronic structure and magnetic properties of the Morse cages are calculated. A series of Mn @A_n (A = Li, Na, K; n = 8-20) clusters are constructed based on a square antiprism. There is a Mn atom in the square antiprism. The structures are constructed by adding A atom one by one. Their stability are predicted and their evolution are studied. How do the evolution of their electronic structure and magnetic properties arise as the cluster size increases is investigated. The results show:(1) In all the cage-like Eu@A_n (A=Li, Na, K, Rb, Cs; n=15-18) clusters, the Eu@Li_n and Eu@Na_n cluster remain the nearly spherical cage structure with higher symmetry.(2) The binding energy and gap of Eu@A_n (A=Li, Na, K, Rb, Cs; n=15-18) become smaller as the atomic weight of alkali metal increase. For each alkali metal series, in addition to Cs, Eu @ A₁₈ clusters have the largest energy gap about 0.5 eV , displaying the characteristic of magic clusters. For Eu @ A₁₆ clusters also display the structural stability and chemical stability, they may be a candidate for magnetic superatom.(3) Between 6.00-10.00μ_B, the magnetic moment of Eu@A_n (A=Li, Na, K, Rb, Cs; n=15-18) is very large, which comes from 4f of. Magnetic moments of Eu @ A₁₈ clusters are 7.00μ_B, which equal to magnetic moment of a europium atom. It reveals that the bonding between 4f electron of Eu and the s electrons of alkali metal is very weak, indicating the magnetic characteristics of superatom. (4) The lowest-lying geometric configuration of Mn@Li₈ is a square anti-prism with D4d system. Eight Li atoms form the anti-primse and a Mn atom at its center. When Lithium atoms add from 8 to 12, the structure evolves from the square anti-prism to the double hooded anti-pentagonal prism with Ih symmetry. Based on the anti-pentagonal prism with double-cap structure, the structure evolves by adding atom individually and becoming closer packing as the sodium atoms from 13 to 20. With extremely high symmetry the structure of Mn@Li₁₂ is very stable.(5) The lowest-lying geometric configuration of Mn@Na₈ is a distorted square antiprism, although the structure of Mn@Na₁₂ is double-capped pentagonal anti- prism with I_h symmetry. Based on the anti-pentagonal prism the structure evolves by adding atom individually becoming closer packing as the Li atoms add from 13 to 20. The magnetic moment of Mn@Na₁₈ clusters is 5.00μ_B, showing the stable super atoms cluster characteristic.(6) The lowest-lying geometric configurations of Mn@K_n (n=8-20) are both with low symmetry and their structure evolution far from the rules. Compared to other alkai metal clusters, the average binding energy is small and the HOMO-LUMO gap is not too large. Their stability is relatively weak. When the number of K atoms n equals to 8, 14, 18, 20, the magnetic moment of these clusters equal to 5.00μ_B.