Extended X-ray Absorption Fine Structure Thermal Factor Analysis of Rattling in Filled Skutterudites RT4Sb12 (R: La, Ce, Pr, Nd, and Sm; T: Fe, Ru, and Os)

A characteristic behavior called rattling has been observed in several filled skutterudites RT4X12 (R: rare earth, T: transition metal, X: pnictogen) by various methods. However, rattling phenomena have not been systematically understood because the characteristics of rattling are different when observed with different measurement methods. We have systematically investigated rattling using X-ray absorption fine structure (XAFS) analysis in order to gain a unified picture of rattling phenomena in filled skutterudites. The temperature dependence of extended X-ray absorption fine structure (EXAFS) thermal factors provides both dynamic and static information. Off-center distances were derived from the static distortions obtained by Einstein-model fitting to the EXAFS Debye–Waller factors. The key relation between the off-center distance, defined by the interatomic distance between the rare-earth and pnictogen, and the cage space, defined by the atomic and ionic radii of the rare-earth and pnictogen, was shown. The off-center distance was found to be approximately proportional to the cage space in the large cage systems (R–T–X combinations). The rattling effect depends upon not only the existence of a potential barrier in the interatomic potentials between the rare-earth ions and pnictogen atoms but also the coherence in the motion of rare-earth ions.