Metal-Encapsulated Fullerenelike and Cubic Caged Clusters of Silicon

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 @Si16 , M=Hf , Zr, and cubic M@Si14 , M=Fe , Ru, Os, caged clusters. The embedding energy of the M atom is ∼12 eV due to strong M-Si interactions that make the cage compact. Bonding in these clusters is predominantly covalent and the highest-occupied--lowest-unoccupied molecular orbital gap is ∼1.5 eV . However, an exceptionally large gap (2.35eV) is obtained for Ti@Si16 Frank-Kasper polyhedron. Interaction between these clusters is weak, making them attractive for cluster-assembled materials