CagelikeSi12clusters with endohedral Cu, Mo, and W metal atom impurities

Abstract

In a recent series of mass-spectrometric ion trap measurements [H. Hiura et al., Phys. Rev. Lett. 86, 1733 (2001)], the formation of silicon clusters with endohedral transition-metal impurities was observed. Particular stability was assigned to the experimentally detected species ${\mathrm{WSi}}_{12}^{+},$ which has been shown by ab initio geometry optimization to adopt the shape of a regular hexagonal ${\mathrm{Si}}_{12}$ prism with the W atom in the center. A similar geometry—namely, a ${\mathrm{Si}}_{12}$ double-chair structure surrounding the metal atom impurity—has emerged from our extensive investigations of silicon clusters in combination with a Cu atom $\left({\mathrm{CuSi}}_N\right)$ as the likely ground-state structure of ${\mathrm{CuSi}}_{12}.$ These results suggest the systematic importance of ${\mathrm{Si}}_{12}$ cages derived from regular structures with $D_{6h}$ geometry for the architecture of silicon clusters containing metal atom impurities. In the present comparative study, we discuss the salient features of endohedral $M{\mathrm{Si}}_{12}$ clusters with $M=\mathrm{Cu},$ Mo, W, as well as several cationic and anionic species of these systems, with regard to their geometric and electronic structure. The interaction between the ${\mathrm{Si}}_{12}$ cage and the enclosed metal impurity is characterized as strongly delocalized bonding for $M=\mathrm{Mo},$ W, while Cu tends to form directed bonds with selected atoms of the cage. Linear extension of the $M{\mathrm{Si}}_{12}$ $(\mathrm{M}\mathrm{e}=\mathrm{M}\mathrm{o},\mathrm{W})$ cells along their principal axes leads to units of the form $M_2{\mathrm{Si}}_{18}.$