Analysis of thermoelectric transport properties of Sb₂Te₃ polycrystalline alloys with in situ formation of CoSb_0.5Te_1.5-based microinclusions via effective medium theory

Sb₂Te₃—a well-known p-type thermoelectric (TE) material with a narrow bandgap and inherently high electrical conductivity—has attracted considerable attention for medium-temperature applications. In this study, the TE performance of Sb₂Te₃-based materials was investigated by introducing Co in Sb₂Te₃ TE alloys as nominal Sb_2-xCo_xTe₃ (x = 0–0.15) compositions. With introducing Co in Sb₂Te₃, CoSb_0.5Te_1.5-based secondary phase with a marcasite-type orthorhombic structure was formed. The formation of this n-type inclusion within the p-type Sb₂Te₃ matrix led to carrier concentration compensation and interfacial carrier scattering, reducing carrier concentration and mobility simultaneously. Additionally, compositional analysis confirmed that Te-excess conditions in Sb₂Te₃ after forming the secondary phase suppressed the formation of native acceptor-like Sb_Te antisite defects in Sb₂Te₃ matrix, further reducing the carrier concentration. To evaluate the impact of the secondary phase on the transport properties, the effective medium theory (EMT) was applied using CoTe₂-Sb₂Te₃ and CoSbTe-Sb₂Te₃ systems as boundary models. While the Seebeck coefficient exhibited reasonable agreement with the EMT predictions, particularly at elevated temperatures, the electrical conductivity and power factor deviated because of the limitations of the EMT due to interfacial scattering. The lattice thermal conductivities were consistently lower than the theoretical values estimated by EMT, which was attributed to additional phonon scattering at the phase boundaries. Since the power factor degradation outweighed the reduction in lattice thermal conductivity, leading to a reduced zT for all the composite samples. However, EMT modeling suggested that optimizing the composition and volume fraction of the inclusion phase could enhance the TE performance. This study provides insights into the interplay between phase microstructure and TE transport in Sb₂Te₃-based composites and offers a practical direction for future design strategies.