Electronic structure, bonding nature, and charge transfer in Ba@Si20 and Si20 clusters:: An ab initio study -: art. no. 155403

First-principles electronic structure calculations are carried out for the Si-20 and endohedral Ba@Si-20 clusters, which appear in the silicon clathrate I and II. We analyze electronic structures, binding energies, charge transfers, and exohedral binding energies of single Si atoms on the Ba@Si-20 clusters, We compare the electronic structures of the Ba@Si-20 cluster with that of the corresponding hollow Si-20 cluster. We find that, unlike the hollow I-h Si-20 cluster which undergoes a structural change, the endohedral Ba atom in the Ba@Si-20 cluster stabilizes the cage structure with C-i symmetry slightly distorted from I-h symmetry. The computed geometrical charge transfer is 2.006e and the physical one is 0.063e for the endohedral Ba atom. The eigenstates of both the clusters can be understood on the basis of a simple model previously used to explain the spherical carbon fullerenes. The charge distribution, corresponded to the highest valence electron state, relates to the sp(3) bonding nature of the cage Si atoms of the Ba@Si-20 cluster. The top positions of the cage Si atoms on this cluster is the most unstable among the positions investigated despite the sp(3) bonding of the cage atoms; the position on the center of edge is the most stable for the exohedral Si atom; the hollow site at the center of the pentagonal ring is the second. The two exohedral positions, top and 6c sites, around the Ba@Si-20 cluster in the clathrate I are the most unstable among the positions. We discuss the mechanism of the growth of the clathrates I.