Evolution of thermoelectric transport properties of Sb₂Te₃–Sb₂Se₃ solid-solution alloys depending on phase formation behavior

Sb₂Te₃-based alloys exhibit decent thermoelectric transport properties in the mid-temperature range above 550 K. However, the study on Sb₂Te₃ alloys has been limited in simple doping approach. Herein, evolution in the thermoelectric transport properties associated with the phase formation of solid solution alloys of Sb₂(Te_1−xSe_x)₃ (x = 0, 0.25, 0.33, 0.5, 0.75, and 1.0) compositions is investigated to widen the strategy to solid solution alloying. A single phase of the Sb₂Te₃ rhombohedral structure formed from x = 0 to 0.5, whereas mixed phases with an orthorhombic structure of Sb₂Se₃ was observed at x = 0.75. The electrical conductivity weighted mobility were gradually decreased as Se content increases to x = 0.75. Consequently, the power factor was decreased gradually and significantly to 0.076 mW/mK² for x = 0.75 (Sb₂(Te_0·25Se_0.75)₃) compared with 2.6 mW/mK² of the pristine sample. The total thermal conductivity of 2.0 W/mK for the Sb₂Te₃ was significantly reduced gradually to 0.64 W/mK for x = 0.75 owing to simultaneous decrease in electrical and lattice thermal conductivities. The reduction in lattice thermal conductivity is mainly owing to the addition point defect scattering caused by the Se addition. Nevertheless, thermoelectric figure of merit zT of Sb₂Te₃ is gradually decreased from 0.64 to 0.11 for x = 0.75 at 650 K due to the degradation in electrical transport properties. Furthermore, the theoretical zT was estimated for all samples with varying carrier concentration based on single parabolic band model.