Multiscale Length Structural Investigation and Thermoelectric Performance of Double-Filled Sr 0.2 Yb 0.2 Co 4 Sb 12 : An Exceptional Thermal Conductivity Reduction by Filler Segregation to the Grain Boundaries.

Among thermoelectric materials, skutterudites are the most prominent candidates in the mid-temperature range applications. In the multiple-filled Sr 0.2 Yb 0.2 Co 4 Sb 12 skutterudite, with Sr and Yb as fillers, we have enhanced the thermoelectric performance of CoSb 3 through the reduction of lattice thermal conductivity and the optimization of carrier concentration and electrical conductivity. The high-pressure synthesis of the double-filled derivative promotes filling fraction fluctuation. This is observed by high angular resolution synchrotron X-ray diffraction, showing a phase segregation that corresponds to an inhomogeneous distribution of the filler atoms, located at the 2 a positions of the cubic space group Im3̅. In addition, scanning transmission electron microscopy (STEM) combined with EELS spectroscopy clearly shows a segregation of Sr atoms from the surface of the grains, which is compatible with the synchrotron X-ray powder diffraction results. Mean square displacement parameters analysis results in Einstein temperatures of ∼94 and ∼67 K for Sr and Yb, respectively, and a Debye temperature of ∼250 K. The strong effect on resonant and disorder scattering yields a significantly lower lattice thermal conductivity of 2.5 W m -1 K -1 at 773 K. Still, good weighed-mobility values were obtained, with high filling fraction of the Yb and Sr elements. This drives a reduced electrical resistivity of 2.1 × 10 -5 Ω m, which leads to a peak zT of 0.26 at 773 K. The analysis and results performed for the synthesized (Sr,Yb)-double filled CoSb 3 , shed light on skutterudites for potential waste-heat recovery applications.