Structural properties and thermal conductivity of crystalline Ge clathrates

¯ n with lattice parameters of 10.721~2! and 10.703~2! A respectively. Single-crystal x-ray diffraction and structural refinement indicate that the randomly distributed Ga and Ge atoms form a tetrahedrally bonded three-dimensional net in whose cavities the ‘‘guest’’ Sr or Eu atoms reside. The ‘‘guest’’ atoms in the smaller of these polyhedra ~dodecahedra! have spherical thermal ellipsoids while those in the larger polyhedra ~tetrakaidecahedra! display relatively large and highly anisotropic thermal ellipsoids. The low-thermal conductivity of these compounds at low temperatures is attributable to the disorder introduced by the dynamic ‘‘rattling’’ introduced by these ‘‘guest’’ atoms inside the polyhedra. The potential of this material system for thermoelectric applications is also discussed.