Einstein modes in the phonon density of states of the single-filled skutterudite Yb0.2Co4Sb12

Measurements of the phonon density of states by inelastic neutron time-of-flight scattering and specific-heat measurements along with first-principles calculations, provide compelling evidence for the existence of an Einstein oscillator (rattler) at omega(E1) approximate to 5.0 meV in the filled skutterudite Yb0.2Co4Sb12. Multiple dispersionless modes in the measured density of states of Yb0.2Co4Sb12 at intermediate transfer energies (14 <= omega <= 20 meV) are exhibited in both the experimental and theoretical density of states of the Yb-filled specimen. A peak at 12.4 meV is shown to coincide with a second Einstein mode at omega(F.2) approximate to 12.8 meV obtained from heat-capacity data. The local modes at intermediate transfer energies are attributed to altered properties of the host CoSb3 cage as a result of Yb filling. It is suggested that these modes are owed to a complementary mechanism for the scattering of heat-carrying phonons in addition to the mode observed at omega(E1) approximate to 5.0 meV. Our observations offer a plausible explanation for the significantly higher dimensionless figures of merit of filled skutterudites, compared to their parent compounds.