Einstein modes in the phonon density of states of the single-filled skutterudite Yb_0.2 Co₄ Sb₁₂

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 ω_E1 ≈ 5.0 meV in the filled skutterudite Yb_0.2 Co₄ Sb₁₂. Multiple dispersionless modes in the measured density of states of Yb_0.2 Co₄ Sb₁₂ at intermediate transfer energies (14≤ω≤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 ω_E2 ≈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 ω_E1 ≈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.