Strongly decoupled europium and iron vibrational modes in filled skutterudites

The europium partial vibrational density of states (DOS) in EuFe₄Sb₁₂ and the iron partial vibrational DOS in both EuFe₄Sb₁₂ and Ce⁵⁷Fe₄Sb₁₂ have been obtained by nuclear inelastic scattering. The results reveal the strong independence of the iron and rare-earth vibrational modes. The cage filling europium only participates significantly to the low-energy local vibrational modes. The force constants have been obtained from the measured probability of nuclear absorption. The energies of the peaks in the europium and iron DOS are in excellent agreement with the calculated DOS in LaFe₄Sb₁₂. The results indicate that nuclear inelastic scattering is the technique of choice for the study of the localized vibrational modes in thermoelectric ``phonon glass'' materials.