Electronic Origin of the Stability of Transition-Metal-Doped B₁₄ Drum-Shaped Boron Clusters and Their Assembly into a Nanotube

Abstract

We study the stability of drum-shaped transition metal (TM)-doped boron clusters, M@B n with n = 14 and 16, and M = 3d, 4d, and 5d TM atom using ab initio calculations. Our results show that drum-shaped M@B 14 clusters are favored for M = Cr, Mn, Fe, Co, and Ni, while in other cases, open conical or bowl shaped structures become more favorable. The isoelectronic Ni@B 14 and Co@B 14 − clusters have large highest occupied molecular orbital−lowest unoccupied molecular orbital gaps and these are magic clusters. Their stability has been correlated with the occurrence of magic behavior with 24 valence electrons in a disk jellium model, while for Fe@B 14 case the drum structure is deformed and the stability occurs at 22 delocalized valence electrons. The bonding nature in these clusters has been studied by analyzing the electron density at bond and ring critical points, the Laplacian distribution of the electron density, the electron localization function, the source function, and electron localization-delocalization indices, all of which suggest two-and threecenter σ bonding within and between the two B 7 rings, respectively, and hybridization between the TM d orbitals and the π bonded molecular orbitals of the drum. The infrared and Raman spectra of these magic clusters show all real frequencies, suggesting the dynamical stability of the drum-shaped structures. There is a low frequency mode associated with the M atom. Results of the electronic spectra of the anion clusters are also presented that may help to identify these species in future experiments. Further, we discuss the stability of 24 delocalized valence electron systems Mn@B 16 anion, Fe@B 16 , Co@B 16 cation, and other related clusters. Assembly of Co@B 14 clusters has been shown to stabilize a carbon nanotube-like nanotube of boron with Co atomic nanowire inside while a nanotube of boron with triangular network has been obtained with the assembly of Fe@ B 16 drum-shaped clusters. Both the nanotubes are metallic.

Smallest Fullerene-like Structures of Boron with Cr, Mo, and W Encapsulation

“…But upon optimization it ends up in an open structure indicating that the formation of a boron cage for this size is unlikely with endohedral doping of an Mo atom. However, recently drum-shaped clusters of 16 boron atoms have been obtained with the doping of a Co atom, while bowl-shaped clusters have been shown [39] to be formed for many of the 3d, 4d, and 5d transition M atoms. Also 14-atom drum-shaped boron clusters have been shown [39] to be favored by doping of some 3d transition M atoms such as Cr, Fe, Co, and Ni.…”

“…[34] The stability of some disk-shaped clusters has been correlated with electronic shell closing at 12 valence electrons. [34] Further, recently bowl-shaped [35,36,37,38,39] and drum-shaped [40,39] structures of boron have been predicted to be stabilized by M atom dopants such as M@B 14 drum with M = Cr, Fe, Co, and Ni, and also with 16 boron atoms such as Co@B − 16 . We have studied M atom encapsulated boron clusters in the size range of 18 to 24 atoms in order to find the smallest cage of free boron clusters besides the drum structures.…”

Structural transformations in boron clusters induced by metal doping

“…However, these two stoichiometries result from different interactions between the orbitals of B 14 and the dopant atom. Xu et al 178,179 analyzed some double-ring B clusters with a TM at the center. They found twelve delocalized orbitals in B 14 Ni, three π-orbitals in the tangential direction filled with six valence electrons following Hückel's (4 n + 2) rule.…”

“…However, such works did not include an exhaustive exploration of the PES, so the reliability of this form being the GM is uncertain. 178,179…”

Exploring the structure and electronic properties of germanium doped boron clusters using density functional theory based global optimization method

“…76,77 For example, Li et al 75 explored the doping of a planar boron cluster B 18 − with a 3d-metal atom Co to gain insight into how dopants can affect the properties of pristine borophenes. Saha et al 78 identified that drum and bowl type clusters become stable on doping transition metals in B 14 clusters. Similarly, Lv and co-workers 68 determined that B 24 upon encapsulation with transition metals transforms into a stable endohedral fullerene structure.…”