Effects of charge compensation on the thermoelectric properties of (La_1-zCe_z)_0.8Fe_4-xCo_xSb₁₂ skutterudites

La/Ce-partially double-filled and Co-charge-compensated (La_1−zCe_z)_0.8Fe_4−xCo_xSb₁₂ skutterudites were synthesized, and their thermoelectric properties were studied by varying the filling ratio and charge compensation. X-ray diffraction analysis revealed that the matrix phase was skutterudite and a secondary phase was determined to the marcasite FeSb₂. However, the formation of marcasite could be inhibited by increasing the Co content. Rare-earth antimonides, including LaSb₂ and CeSb₂, which were formed in fully filled La_1−zCe_zFe_4−xCo_xSb₁₂, were not found after La/Ce partial filling. La/Ce filling and Co substitution were confirmed by the decrease in lattice constants, from 0.9137 to 0.9099 nm, with increasing Ce and Co contents. Electrical conductivity showed negative temperature dependence, indicating metallic or degenerate semiconductor characteristics. Intrinsic conduction resulted in the maximum Seebeck coefficient at temperatures between 723 and 823 K. As the Co-substitution and Ce-filling contents increased, the Seebeck coefficient increased, while electrical and thermal conductivities decreased. This was considered to be due to difference in the valences of La³⁺ and Ce^3+/4+ and the increase in carrier concentration caused by Co charge compensation. However, because they had similar atomic masses and ionic radii, the effects of the La/Ce filling ratio were not significant. Instead, Co charge compensation had the dominant effect on thermoelectric properties. The maximum Seebeck coefficient of 165.4 µVK^-1 was obtained for (La_0.25Ce_0.75)_0.8Fe₃CoSb₁₂ at 823 K, and the highest electrical conductivity of 2.27 × 10⁵ S m^-1 was achieved for (La_0.75Ce_0.25)_0.8Fe₄Sb₁₂. (La_0.25Ce_0.75)_0.8Fe₃CoSb₁₂ exhibited the lowest thermal conductivity of 2.15 W m^-1K^-1 at 523 K and (La_0.75Ce_0.25)_0.8Fe_3.5Co_0.5Sb₁₂ showed the highest power factor of 2.53 mW m^-1 K^-2 at 723 K. The maximum dimensionless figure of merit, ZT_max = 0.71, was achieved at 723 K for (La_0.75Ce_0.25)_0.8Fe₃CoSb₁₂