Exohedral bonding nature of Si atom on the Ba@Si28 cluster;: Ab initio study

First-principle electronic structure is studied for the Si-28 and Ba @ Si-28 clusters. which are components of the clusters in silicon clathrate II. We obtain the geometrically optimized relaxed cage structures of the clusters and the exohedral binding nature of single Si atom on the Ba @ Si-28 clusters. The hollow Si-28 cluster relaxes into the Si-24 like cluster in the clathrate I having four exohedral Si atoms outside the hexagon in the Si-24 cluster. The hexagons on the Ba @ Si-28 cluster are deformed into a chair type with relaxation. The exohedral Si atom is the most stable at the edge center near the top that meets three pentagons, We have found for the first time that the exohedral Si atom forms the three-center covalent bond between the two caged Si atoms.