High thermoelectric figure of merit in well optimized YbyCo4Sb12

By implementing X-ray absorption spectroscopy among other techniques, ytterbium was inferred to display a valence of 2 in the thermoelectric YbyCo4Sb12 skutterudites (0.10 ≤ y ≤ 0.20). Consequently, the correlation PF(A3+) < PF(A2+) < PF(A1+) between the valence of the filler atom and the power factor (PF) could be established in a generic set of AyCo4Sb12 skutterudites (A = filler atom and electron donor). By grinding the YbyCo4Sb12 samples under inert atmosphere prior to densification by Spark Plasma Sintering, no spurious oxidation of Yb occurred, conversely to what happens when grinding in air. This led to the effective ytterbium concentration being strictly equal to the nominal one, up to the solubility limit determined to be ylim = 0.41 ± 0.02 mol per formula unit when the YbyCo4Sb12 compounds are synthesized at 1073 K. The maximum PF = 4.2 mW m−1 K−2 at 300 K is reached in Yb0.15Co4Sb12, for the first time at the theoretically predicted electron concentration nopt = 1.8 ± 0.2 × 1020 cm−3, thus evidencing the optimization of this compound. Finally, Yb0.15Co4Sb12 also displays the best thermoelectric properties in the series, with a figure of merit ZTmax = 1.4 and PF = 5.5 mW m−1 K−2 at 750 K.