Synthesis and Thermoelectric Properties of Charge-Compensated SyPdxCo4–xSb12 Skutterudites

Recently, the electronegative elements (e.g., S, Se, Cl, and Br) filled skutterudites have attracted great attention in thermoelectric community. Via doping of some electron donors at the Sb sites, these electronegative elements can be filled into the voids of CoSb3 forming thermodynamically stable compounds, which greatly extends the scope of filled skutterudites. In this study, we show that doping appropriate elements at the Co sites can also stabilize the electronegative elements in the voids of CoSb3. A series of SyPdxCo4-xSb12 compounds were successfully fabricated by a traditional solid state reaction method combined with a spark plasma sintering technique. The phase composition and electrical and thermal transport properties were systematically characterized, and the related mechanisms were deeply discussed. It is found that the charge compensation between Pd doping and S filling is the main reason for the formation of thermodynamically stable SyPdxCo4-xSb12 compounds. Filling S element in the voids of CoSb3 provides additional holes to reduce the carrier concentration while scarcely affecting the carrier mobility. However, doping Pd at the Co sites not only changes the carrier scattering mechanism but also deteriorates the carrier mobility. Low lattice thermal conductivities are observed in these SyPdxCo4-xSb12 compounds, which are attributed to the low resonant frequency of the S element. Finally, a maximal figure of merit of 0.85 is obtained for S0.05Pd0.25Co3.75Sb12 at 700 K.