Strongly reduced lattice thermal conductivity in Sn-doped rare-earth (M) filled skutterudites M x Co 4 Sb 12- y Sn y , promoted by Sb-Sn disordering and phase segregation.

CoSb 3 thermoelectric skutterudite has been filled with rare-earth metals (M = La, Ce, Yb) and partially doped with Sn in specimens of M x Co 4 Sb 12- y Sn y stoichiometry. This has been achieved under high-pressure conditions at 3.5 GPa in a piston-cylinder hydrostatic press. A structural investigation using synchrotron X-ray diffraction data reveals a phase segregation in twin skutterudite phases with filling fraction fluctuation and different unit-cell sizes. As a result of three effects acting as phonon scatterers, namely the rattling effect of M at the wide 8 a cages of the cubic Im 3̄ structure, the phase segregation, and the intrinsic disorder introduced by Sn substitution at the Sb sublattice, the total thermal conductivity ( κ ) dramatically falls to reach minimum values under 2 W m -1 K -1 , well below those typically exhibited by other thermoelectric materials based upon single-filled skutterudites. The power factor is substantially enhanced to 1.11 mW m -1 K -2 in Yb 0.5 Co 4 Sb 11.6 Sn 0.4 with respect to the unfilled composition, as a result of the charge transfer promoted by the filler.