Electronic structures of Na8Si46 and Ba8Si46

The effects of Na and Ba atoms doped into silicon clathrate compounds, Na8S i46 and Ba8Si46, on their energy-band modification have been studied. Both Na and Ba atoms occupy center sites of dodecahedral (Si-20) and tetrakaidec ahedral cages (Si-24), irrespective of the structure of clathrate compounds . Their electronic structures are calculated within the framework of densit y functional theory. In Na8Si46 clathrate, the Na state is weakly hybridize d with the Si-46 conduction-band state. This weak hybridization results in almost rigid energy-band modification of pristine Si-46. In Ba8Si46 clathra te, the conduction band is strongly modified by the Ba state. The Fermi lev el of Ba8Si46 is located closely to a strong peak of the density of states at the conduction-band edge. Such modification causes the superconductive n ature observed in the silicon clathrate doped with Ba.