In the filled gallium-germanium clathrates, R8Ga16Ge30, where R is Ba, Sr, or Eu, the guests are located in two large cages and are weakly bound to the crystalline clathrate framework. The caged guests exhibit a localized “rattling” vibrational mode that provides an efficient mechanism for reducing the thermal conductivity. Inelastic neutron scattering and nuclear inelastic scattering measurements have yielded the phonon density of states in R8Ga16Ge30; the line width of the localized vibrational modes is found to be an important parameter in determining the lattice thermal conductivity. Neutron diffraction studies on R8Ga16Ge30 have shown that the guests in the larger cage are located off-center, and it was proposed that their jumping about the four off-center locations is responsible for the observed glass-like thermal conductivity at temperatures below 10 K. The detection of such slow guest motion is challenging because the typical time and energy scales involved are ca. 4 ns and 1 µeV, respectively. We have studied the slow europium tunneling dynamics in Eu4Sr4Ga16Ge30 by both Mössbauer and microwave absorption spectroscopy.

Resonant Ultrasound Spectroscopy and low-temperature ultrasonic attenuation measurements are reported for filled and unfilled skutterudites and for Ge-clathrates. These data reveal that an unusual elastic behavior complements the thermal properties of the filled skutterudites and indicate the presence of low-energy vibrational modes. The attenuation at low-temperatures in the single-crystalline Ge-clathrate is glasslike and can be described by the same phenomenological Tunneling Model that has been developed to describe the low-temperature properties of amorphous solids.

We present initial assessments of the thermoelectric properties of two ternary tellurides with known crystal structures, Tl2GeTeM5 and Tl2SnTe5. Tl2SnTe5 appears to have a p-type figure of merit about the same as that of Bi2Te3, the best thermoelectric material among binary compounds. A good figure of merit is possible because the lattice thermal conductivity is very low. Based on neutron diffraction data, we have calculated atomic displacement parameters and thermal expansion coefficients. The atomic displacement parameters give some understanding of the low lattice thermal conductivity.

Polycrystalline CoSb3 films were grown on a variety of electrically insulating substrates by pulsed laser ablation from a stoichiometric hot-pressed target. These films are fully crystallized in the skutterudite structure, and the grains exhibit a strongly preferred alignment of the cubic [310]-axis perpendicular to the substrate surface. The film quality is studied for different single-crystal substrates and as a function of growth temperature and background gas.

Hall measurements show that the films are p-type semiconducting with a room-temperature carrier density of 3×1020 holes/cm3. The Hall mobility is found to be 50 to 60 cm2 /Vs, which is high for such a heavily-doped material. The Seebeck coefficient and the resistivity are measured as a function of temperature and are compared to bulk measurements.