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@Si(16), M = Hf, Zr, and cubic M@Si(14), M = Fe, Ru, Os, caged clusters. The embedding energy of the M atom is approximately 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 approximately 1.5 eV. However, an exceptionally large gap (2.35 eV) is obtained for Ti@Si(16) Frank-Kasper polyhedron. Interaction between these clusters is weak, making them attractive for cluster-assembled materials.