Phonon Density of States in Equilibrium and Non-equilibrium Prepared Ytterbium and Cerium-Filled Skutterudites

Filled skutterudites have been extensively studied due to their high thermoelectric figure of merit (ZT) and promise for applications in the 400 K -- 800 K range. They tend to exhibit higher ZT values than their binary counterparts, and a so-called ``rattling effect'' has been suggested to be a major factor that effectively reduces lattice thermal conductivity. However, this ``rattling'' scenario for the vibrational modes of a heavy element in a skutterudite cage remains controversial. We measured the phonon density of states (DOS) of Ce and Yb-filled skutterudites prepared by conventional equilibrium and non-equilibrium synthesis methods via inelastic neutron scattering. Nanoscaled substructures are commonly observed in the non-equilibrium samples as examined by high resolution TEM. Preliminary data analysis shows that the Yb filled antimonides exhibit well-defined localized modes in the 4-12 meV range and several unresolved optical modes in the 30-35 meV range, as compared with their Ce-filled counterparts. Differences in the 3-10 meV range in the DOS of the latter are believed to arise from non-equilibrium structures that developed during preparation.