"Stuffed" Skutterudite Antimonides as New Thermoelectric Materials. B. C. Chakoumakos, B. C. Sales, D. G. Mandrus, Solid State Division, Oak Ridge National Laboratory*, Oak Ridge, TN 37831-6393.

The ultimate thermoelectric material should conduct electricity like a crystal, but conduct heat like a glass. Ternary compositions of the title compounds, such as LaFe₃CoSb₁₂, meet many of the requirements for good thermoelectric materials, so the structural chemistry is being tailored to optimize the essential transport properties (Science 272, 1996, 1325; Physics Today, March 1997, 42). For the skutterudite structure the M₄Sb₁₂ corner-linked octahedral framework dominates the band structure and therefore the electronic transport. The lanthanide atoms are loosely stuffed into oversized framework cages and effectively scatter heat-carrying phonons, thereby reducing the thermal conductivity. The thermoelectric figure of merit, ZT, is near 1 at 800K. Neutron diffraction studies using powdered and single-crystal samples are underway at the High Flux Isotope Reactor, to measure the temperature dependence of the structural parameters. Preliminary results show that the atomic displacement parameters (ADP) for the (Fe,Co) and Sb atoms have typical values and are proportional to T, but for La this is not true. The La ADP is 5 time greater even at 10K. Unconstrained refinement of both the La site occupation and the La ADP, reduces the site occupation to 0.75 and the ADP by one half. A variable La site occupation is tied to the Fe:Co ratio, but in this case it refines to the stoichiometric value of 3:1.

*Supported by the Division of Materials Sciences, U.S. D.O.E. (contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corporation).