Enhanced Thermoelectric Performance in Ternary Skutterudite Co(Ge_0.5Te_0.5)₃ via Band Engineering

We report the phase evolution and thermoelectric properties of a series of Co(Ge_0.5Te_0.5)_3-Sb (x = 0-0.20) compositions synthesized by mechanical alloying. Pristine ternary Co(Ge_0.5Te_0.5)₃ skutterudite crystallizes in the rhombohedral symmetry (R3̅), and Sb doping induces a structural transition to the cubic phase (ideal skutterudite, Im3̅). The Sb substitution increases the carrier concentration while maintaining a high thermopower even at higher doping levels owing to an increased effective mass. The exceptional electronic properties exhibited by Co(Ge_0.5Te_0.5)₃ upon doping are attributed to the carrier transport from both the primary and secondary conduction bands, as shown by theoretical calculations. The enhanced electrical conductivity and high thermopower increase the power factor by more than 20 times. Because the dominant phonon propagation modes in binary skutterudites are associated with the vibrations of pnictogen rings, twisting the latter through the isoelectronic replacement of Sb₄ rings with Ge₂Te₂ ones, as done in this study, can effectively reduce the thermal conductivity. This leads to an increase in the dimensionless figure-of-merit (zT) by a factor of 30, reaching 0.65 at 723 K for Co(Ge_0.5Te_0.5)_2.9Sb_0.1.