Extended x-ray absorption fine structure (EXAFS) measurements have been carried out at the $L$ $K$- or $L_{\mathrm{III}}$-edges ($L=\mathrm{Ce}$, Eu, and Yb) and $T$ $K$-edges ($T=\mathrm{Fe}$ and Ru) for a series of filled skutterudite materials, $L{T}_4{X}_{12}$ ($X=\mathrm{P}$ and Sb). The high correlated Debye temperature $({\Theta }_D\sim 400\mathrm{K})$ obtained for the $T\text{−}X$ peak indicates that the $T_4{X}_{12}$ framework is relatively stiff. In contrast, the low Einstein temperature $({\Theta }_E\sim 100\mathrm{K})$ obtained for the $L\text{−}X$ or $T\text{−}L$ 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 $\mathrm{Ce}{\mathrm{Os}}_4{\mathrm{Sb}}_{12}$ than in $\mathrm{Ce}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$. 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 ${\sigma }^2\left(T\right)$ 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.

• Received 24 October 2003

DOI:https://doi.org/10.1103/PhysRevB.70.094109