Metal-Encapsulated Fullerenelike and Cubic Caged Clusters of Silicon

Abstract

We report metal-encapsulated caged clusters of silicon from ab initio pseudopotential plane wave calculations using generalized gradient approximation for the exchange-correlation energy. Depending upon the size of the metal ( $M$) atom, silicon forms fullerenelike $M@{\mathrm{Si}}_{16}$, $M=\mathrm{Hf}$, Zr, and cubic $M@{\mathrm{Si}}_{14}$, $M=\mathrm{Fe}$, Ru, Os, caged clusters. The embedding energy of the $M$ atom is $\approx 12\mathrm{eV}$ due to strong $M-\mathrm{Si}$ interactions that make the cage compact. Bonding in these clusters is predominantly covalent and the highest-occupied–lowest-unoccupied molecular orbital gap is $\approx 1.5\mathrm{eV}$. However, an exceptionally large gap (2.35 eV) is obtained for $\mathrm{Ti}@{\mathrm{Si}}_{16}$ Frank-Kasper polyhedron. Interaction between these clusters is weak, making them attractive for cluster-assembled materials.