Properties of semiconductor clathrates from density functional theory

We have performed density functional calculations of structural, electronic and vibrational properties of Ge-based clathrate materials. Variants of these clathrates have been shown to be promising thermoelectric materials. We have incorporated all the elements of Column I and Column II in the cages of (Ga/sub x/Ge/sub 1-x/)/sub 46/ clathrates as cation guests, and have examined their "rattling" behavior. The potential energy curves of guests in the cages are determined to understand the nature of guest-framework interactions. Some atoms are unstable at the center of the cage, while others appear to have "rattler" behavior. The distribution of Ga atoms influences the energetic and electronic properties of clathrate systems. The effects of substitutional Ga and of the encapsulated cations on band-structures are discussed.