High thermoelectric figure of merit predicted in Cu26V2Sn6Se32 colusite induced by vacancy defects and glassy-like vibrational modes

High-performance thermoelectric (TE) properties of pristine Sn-based colusites are investigated theoretically. A recent experimental article [Bourgès et al., J. Am. Chem. Soc. 140, 2186 (2018)] showed how structural disordering reduced the lattice thermal conductivity in Cu26V2Sn6S32 and improved the TE figure of merit (ZT). In this article, it is observed that low energy soft optical phonons and the vacancy defect play a crucial role in reducing thermal conductivity. An ultra-low lattice thermal conductivity 0.35Wm−1K−1 is observed in Cu26V2Sn6Se32. It is seen that mass variance perturbation and number of vacancies are important to tune the thermal and electrical properties. Though the electrical conductivity decreases with the number of vacant sites, the Seebeck coefficient enhances due to the reduction in carrier concentration. The detailed study of thermoelectric properties of Cu26V2Sn6Se32 with Se vacancies points to enhanced ZT values of 0.34 at 300 K, which is ∼15 times greater than ZT of Cu26V2Sn6S32. A maximum ZT of 1.68 at 755 K is predicted for Cu26V2Sn6Se32 with Se vacancies, which is the highest to date reported for Sn-based colusites.