LOCALIZED VIBRATIONAL-MODES IN METALLIC SOLIDS

Filled skutterudite antimonides(1,2) are cubic compounds with the form ula RM4Sb12, where R is a rare-earth element (such as La or Ce), and M is a transition metal (for example, Fe or Co). The rare-earth ion is weakly bound in an oversized atomic cage formed by the other atoms. It s presence has been shown to cause a dramatic reduction in the lattice component of the thermal conductivity, while having little effect on the electronic properties(3-5) of the compound. This combination of pr operties makes filled skutterudites of interest as thermoelectric mate rials. It has been suggested(4) that localized, incoherent vibrations of the rare-earth ion are responsible for the reduction in thermal con ductivity, but no direct evidence for these local vibrational modes ex ists. Here we report the observation of local modes in La-filled skutt erudites, using heat capacity, elastic constant and inelastic neutron scattering measurements. The La atoms show unusual thermodynamic behav iour, characterized by the presence of two low-energy localized modes. Our results suggest that consideration of local modes will play an im portant role in the design of the next generation of thermoelectric ma terials.