Structures and Stabilities of Clusters of Si12, Si18, and Si20 Containing Endohedral Charged and Neutral Atomic Species

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Electronic structure calculations based on density functional theory and Møller Plesset perturbation theory were performed on three isomers of Sin and on the endohedral clusters Si12 containing neutral or charged atomic species. The existence of endohedral clusters depends on the Si 12 cage shape and the nature of the embedding species. Endohedral clusters of Li0,1,-1, Na0,1,-1, and He in Si12 cages were found. In contrast, K+, Ne, F-, or Cl - do not form endohedral clusters with Si12 due to their large size. All endohedral clusters that are minima on the potential energy surfaces are stable and have large HOMO-LUMO gaps (>1 eV). The stability order for the lithium and sodium clusters is: anionic clusters > neutral clusters > cationic clusters. The endohedral complex of two Li atoms with the Si18 cage is lower in energy than the sum of the empty Si 18 cage and two Li atoms. In contrast, doping two Na atoms into the Si18 cage forms an exohedral Na2Si18 cluster. An endohedral cluster of Li2 with Si20 was also investigated and characterized. The stability of the endohedral complexes of two Li atoms in Si18 and Si20 suggest that silicon nanotubes, which are unstable, might be stabilized by an internal string of Li atoms.