W0130

Anisotropic Displacement Parameters in Filled Skutterudites. B.C. Chakoumakos, B.C. Sales, D.G. Mandrus, V. Keppens, Solid State Division, Oak Ridge National Laboratory^*, Oak Ridge, TN 37831-6393 USA

Quaternary skutterudites, such as LnFe_3-xCo_xSb₁₂ where Ln = lanthanide, have enhanced thermoelectric efficiency in part due to their glass-like thermal conductivity. Mean square atomic displacements in nominally LaFe₃CoSb₁₂, determined as a function of temperature using neutron diffraction of a single crystal, show that the La atom exhibits an anomalously large displacement at room temperature, U_eq = 0.0196(9) Ų, due to its undersized filling of the atomic cage formed by the corner-linked octahedral framework of M ₄Sb₁₂, M= Fe,Co. Site occupancy refinements show 25% vacancies on the La site and an actual Fe/Co ratio of 2.17. Analysis of the temperature dependence of the atomic displacements identifies a significant temperature independent component for the La atom ascribed to static disorder, which amounts to 19% of the room temperature value. The large amplitude rattling of the La atom is linked to the dramatic decrease of the lattice contribution to the thermal conductivity, which is a key ingredient for improving the thermoelectric behavior of these materials.   For the Ce composition the transport properties are different so an analysis of its ADP behavior is being made for comparison. Neutron diffraction is ideal for this system because of the low absorption, excellent scattering length difference between Fe and Co, and the overall sensitivity to nuclear displacements.

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