L. Girard¹, R. Viennois¹, L. Chapon², A. Haidoux¹, H. Mutka³, J-C. Tedenac¹, D. Ravot¹

The insertion of heavy elements (rare earth) in the large voids of the skutterudites structure (MSb₃) produces a drastic reduction of the thermal conductivity (k) of theses materials without significantly affecting the electrical conductivity making these compounds good thermoelectric candidates. The point is to understand how these two contradictory phenomenons occur in order to improve these materials. The reduction of k by the interaction of the rare earth "rattling" with some Sb phonon branches, was evidenced. But the fact that k and the filling fraction of rare earth (R) are not linearly related with a minimum of k around 50% in spite of a constant frequency of the R oscillator, proves that other mechanisms must be involved in the partially filled sample. To explain this phenomenon, 9 samples of the series Ce_y(Fe-Ni)₄Sb₁₂ (10-300K) were studied by neutron powder diffraction at ISIS. Inelastic neutron scattering were made on CeFe₄Sb₁₂ and LaFe₄Sb₁₂ at ILL. Our measurement of atomic displacement parameters (ADP) demonstrate the existence of a R site static disorder which could broaden the low energy vibrational mode, increasing the effect of the interaction R’s low frequency vibration-Sb phonon branches on k.

Lattice constant and ADP are analysed with Debye and Einstein models and compared with the data of the inelastic measurements. The results are discussed together with the influence of this additional static disorder on the phonon spectra.