Evidence for rattling behavior of the filler atom (L) in the filled skutterudites LT4X12 (L = Ce, Eu, Yb; T = Fe, Ru; X = P, Sb) from EXAFS studies -: art. no. 094109

Extended x-ray absorption fine structure (EXAFS) measurements have been carried out at the L K- or L-III-edges (L=Ce, Eu, and Yb) and T K-edges (T=Fe and Ru) for a series of filled skutterudite materials, LT4X12 (X=P and Sb). The high correlated Debye temperature (Theta(D)similar to400 K) obtained for the T-X peak indicates that the T4X12 framework is relatively stiff. In contrast, the low Einstein temperature (Theta(E)similar to100 K) obtained for the L-X or T-L pairs strongly supports the concept of a rattling local mode behavior for the L ions. The analysis also indicates that this rattling frequency is much smaller in the antimonide skutterudites than in the phosphide ones, and smaller in CeOs4Sb12 than in CeFe4Sb12. Both results indicate that the larger the void within which the Ce atom is located, the lower the rattling frequency. In addition, for some systems in which the signal-to-noise for sigma(2)(T) is high, a fit can be made to extract the reduced mass of the rattling atom; in these cases the obtained reduced mass is close to but slightly below that of the rattler atom mass-clear evidence of a localized mode inside a stiff but not rigid cage. Finally, no clear evidence for any off-center displacement of the filler ions was found in these materials.