Chemical Reactivity and Covalent/Metallic Bonding of Si + over n (n = 11 - 25) Clusters.

Using our thermodynamic force field previously developed to fit the equations of state of bulk Si phases and average coordination numbers of small Si sub n clusters (n = 3-10), we have calculated the equilibrium structures of medium-size Si sub n clusters (n = 11-25). We find the very surprising result that beginning at n=7, near n=13 and near n=19, these clusters follow a pentagonal growth pattern (icosahedra plus associated face capping). Our results are fully consistent with "magic numbers" recently found in the reaction rates for addition of first C sub 2 H sub 4 molecules to Si + over n clusters.