Electronic structures of Na8Si46 and Ba8Si46

The effects of Na and Ba atoms doped into silicon clathrate compounds, Na8Si46 and Ba8Si46, on their energy-band modification have been studied. Both Na and Ba atoms occupy center sites of dodecahedral (Si-20) and tetrakaidecahedral cages (Si-24), irrespective of the structure of clathrate compounds. Their electronic structures are calculated within the framework of density functional theory. In Na8Si46 clathrate, the Na state is weakly hybridized with the Si-46 conduction-band state. This weak hybridization results in almost rigid energy-band modification of pristine Si-46. In Ba8Si46 clathrate, the conduction band is strongly modified by the Ba state. The Fermi level of Ba8Si46 is located closely to a strong peak of the density of states at the conduction-band edge. Such modification causes the superconductive nature observed in the silicon clathrate doped with Ba.