Synthesis and Thermoelectric Properties of La1−zYbzFe4−xNixSb12 Skutterudites

p-Type La1−zYbzFe4−xNixSb12 (0.25 ≤ z ≤ 0.75 and 0.25 ≤ x ≤ 0.5) skutterudites were prepared by encapsulated melting and hot pressing, and the effects of filling and substitution (charge compensation) on the transport and thermoelectric properties were examined. The single-phase skutterudite was successfully synthesized through La/Yb double filling and Ni substitution for Fe. However, a small amount of FeSb2 was detected for La0.25Yb0.75Fe3.75 Ni0.25Sb12, which was attributed to insufficient charge compensation due to deficiency of substitution of Ni for Fe, as well as higher filling fraction of Yb with lower valence than La. The carrier concentration and electrical conductivity decreased with increasing La and Ni contents, with La serving to provide more excess electrons and Ni mediating charge compensation. The electrical conductivity decreased with increasing temperature, indicating that La1−zYbzFe4−xNixSb12 skutterudites were degenerate semiconductors. The positive values of the Hall coefficient and the Seebeck coefficient meant that the major carriers were holes with p-type conduction. The Seebeck coefficient increased with increasing temperature, showing peak values ranging from 723 K to 823 K, which increased with increasing La and Ni contents. The maximum power factor of 2.9 mW m−1 K−2 was obtained for La0.75Yb0.25 Fe3.75Ni0.25Sb12. The thermal conductivity increased at temperatures above 623 K due to bipolar conduction, and the lattice thermal conductivity decreased with increasing filling fraction of Yb, indicating that the Yb filling was more effective at reducing the lattice thermal conductivity than the La filling. The electronic thermal conductivity decreased with increasing La and Ni contents due to the decrease in the carrier concentration. The dimensionless figure of merit (ZT) showed peak values at 723 K, attributed to the decreased Seebeck coefficient (or power factor) and the increased thermal conductivity due to bipolar conduction induced by intrinsic excitation. The maximum ZT = 0.77 was achieved at 723 K for La0.75Yb0.25Fe3.25Ni0.75Sb12.