Einstein Modes in the Phonon Density of States of the Single-Filled Skutterudite Yb₀.₂Co₄Sb₁₂

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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 ωₑ₁ ≈ 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 meV ≤ 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 ωₑ₂ ≈ 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 ωₑ₁ ≈ 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.