Transport properties of polycrystalline Ge clathrates with general composition Sr 8 Ga 16 Ge 30 are reported in the temperature range 5 KрTр300 K. These compounds exhibit N-type semiconducting behavior with relatively high Seebeck coefficients and electrical conductivity, and room temperature carrier concentrations in the range of 10 17-10 18 cm Ϫ3. The thermal conductivity is more than an order of magnitude smaller than that of crystalline germanium and has a glasslike temperature dependence. The resulting thermoelectric figure of merit, ZT, at room temperature for the present samples is 1 4 that of Bi 2 Te 3 alloys currently used in devices for thermoelectric cooling. Extrapolating our measurements to above room temperature, we estimate that ZTϾ1 at T Ͼ700 K, thus exceeding that of most known materials.

The thermal conductivity of polycrystalline semiconductors with type-I clathrate hydrate crystal structure is reported. Ge clathrates (doped with Sr and/or Eu) exhibit lattice thermal conductivities typical of amorphous materials. Remarkably, this behavior occurs in spite of the well-defined crystalline structure and relatively high electron mobility (∼ 100 cm 2 /Vs). The dynamics of dopant ions and their interaction with the polyhedral cages of the structure are a likely source of the strong phonon scattering.

Polycrystalline samples of antimonides with the skutterudite crystal structure with La partially filling the voids have been prepared in an effort to quantify the impact of partial void filling on the lattice thermal conductivity of these compounds. It is observed that a relatively small concentration of La in the voids results in a relatively large decrease in the lattice thermal conductivity. In addition, the largest decrease in the lattice thermal conductivity, compared to ''unfilled'' CoSb 3 is not observed near 100% filling of the voids with La, as was previously believed. This suggests a point-defect-type phonon scattering effect due to the partial, random distribution of La in the voids as well as the ''rattling'' effect of the La ions, resulting in the scattering of a larger spectrum of phonons than in the case of 100% filling. An additional benefit of partial filling in thermoelectric materials is that it may be one way of adjusting the electronic properties of these compounds. Seebeck, resistivity, Hall effect and structural data for these skutterudite compounds are also presented.

Structural analysis and thermal conductivity data on Sr 8 Ga 16 Ge 30 and Eu 8 Ga 16 Ge 30 crystals are reported. These compounds form in the cubic space group Pm3 n with lattice parameters of 10.721͑2͒ and 10.703͑2͒ Å 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 ͑dodecahedra͒ have spherical thermal ellipsoids while those in the larger polyhedra ͑tetrakaidecahedra͒ 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.

Using resonant x-ray spectroscopies combined with density functional calculations, we find an asymmetric biaxial strain-induced d-orbital response in ultrathin films of the correlated metal LaNiO3 which are not accessible in the bulk. The sign of the misfit strain governs the stability of an octahedral "breathing" distortion, which, in turn, produces an emergent charge-ordered ground state with an altered ligand-hole density and bond covalency. Control of this new mechanism opens a pathway to rational orbital engineering, providing a platform for artificially designed Mott materials.