Enhancement in the thermoelectric performance of colusites Cu26A2E6S32 (A = Nb, Ta; E = Sn, Ge) using E-site non-stoichiometry
Colusite-based materials have attracted significant interest in the field of thermoelectrics because of their earth-abundant elements (Cu, S) and high thermoelectric performance. In this study, we demonstrate the enhancement of the thermoelectric figure of merit ZT in colusites Cu26A2E6−xS32 (A = Nb, Ta; E = Sn, Ge; x = 0, 0.5) by modifying their chemical composition. Colusite samples were prepared by melting a mixture of their constituent elements in evacuated quartz tubes followed by hot pressing. The electrical resistivity decreased with Sn and Ge contents, leading to an improvement in the power factor. Energy-dispersive X-ray spectroscopy analysis revealed cation-rich compositions in all the colusite samples. The extra cations were most likely formed during the sintering processes, and they effectively scattered heat-carrying phonons, yielding a low total thermal conductivity (<0.80 W K−1 m−1). For Cu26Ta2Sn6−xS32, scanning electron microscopy analysis revealed the insertion of CuS- and Cu2S-based microscale precipitates, which further reduced the lattice thermal conductivity. At 670 K, a ZT of ∼1.0 was achieved in Cu26Ta2Sn5.5S32, arising from a power factor of ∼800 μW K−2 m−1. Moreover, the low total thermal conductivity (∼0.47 W K−1 m−1 at 670 K) in Cu26Nb2Ge6.0S32 leaded to a high ZT of ∼1.0 at 670 K.