Structural properties and thermal conductivity of crystalline Ge clathrates

Structural analysis and thermal conductivity data on ${\mathrm{Sr}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ and ${\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ crystals are reported. These compounds form in the cubic space group $\mathrm{Pm}3\ifmmode\bar\else\textasciimacron\fi{}n$ with lattice parameters of 10.721(2) and 10.703(2) \AA{} 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.