Thermoelectric properties of p-type filled skutterudites S_xCo₄Sb₁₂

The electronegativity difference between the filler and the host atom of skutterudites determines the bonding type, which effects the thermoelectric properties. The electropositive elements with low electronegativity compared to Sb atom can form n-type filled skutterudites. In this study, the electronegative element sulphur has filled into voids. A series of SxCo₄Sb₁₂ (x= 0.05 to 0.4) compounds were synthesized by the solid-state method. CoSbS secondary phase was observed for x >0.05. Filling fraction of sulphur into the void (∼ 0.04) was calculated using electron probe microanalyzer which agrees with Rietveld refinement of the X-ray diffraction pattern. The transport properties were measured from 350 K to 723 K. The positive Seebeck coefficient (S) confirms p-type semiconducting nature. S and the electrical resistivity (ρ) decreased with increasing x due to increased carrier concentration (n). All the samples except x=0.05 possess almost same S and ρ due to saturation of filling fraction. The enhanced phonon scattering in S_xCo₄Sb₁₂ resulted in low thermal conductivity (κ). The highest zT of 0.18 at 623 K was achieved for S_0.15Co₄Sb₁₂.