UC Santa Cruz

The local structure of two skutterudite families, CeRu₄As₁₂ (arsenides) and LnCu₃Ru₄O₁₂ (Ln = La, Pr, and Nd) (oxy-skutterudites) - have been studied using the Extended X-Ray Absorption Fine Structure (EXAFS) technique. We focus on the lattice vibrations about the rare earth ``rattler" atoms, and the extent to which the vibrations can be considered local modes, with the rattler vibrating inside a nearly rigid cage. In the oxy-skutterudites, in contrast to the As skutterudites, we find the unusual situation that the second neighbor spring constants are stronger than the first. To incorporate these results, and suitably describe the complex rattling behavior in the skutterudites, we present a four mass, one dimensional spring model which contains the essential physics of the the vibrating "rattler" atom, interacting with the cage of surrounding atoms. This model can qualitatively explain the difference between the As skutterudites and oxyskutterudites. Low energy optical modes are identified as the rattling modes and are coupled to the acoustic mode. This coupling reduces the group velocity and is likely the explanation for the low thermal conductivity of the skutterudites.