In recent years, thermoelectric materials inspired from the natural mineral colusite have emerged as a new class of environmentally-friendly copper-based sulfides composed of abundant elements. Herein, high performance bulk colusite Cu 26 V 2 Sn 6 S 32 materials were synthesized using mechanical alloying and spark plasma sintering of low-cost industrial-grade metal sulfides. This new synthesis route has led to the formation of various types of nano-tomicroscale defects, from local Sn-site structural disorder to nano-inclusions and vanadiumrich core-shell microstructures. These multiscale defects have a strong impact over phonon 2 scattering, making it possible to reach ultra-low lattice thermal conductivity. Simultaneously, the electrical transport properties are impacted through variations in charge carrier concentration and effective mass, leading to a synergistical improvement of both electrical and thermal properties. The resulting power factor, over 1 mW m -1 K -2 above 623 K with an average value of 0.86 mW m -1 K -2 over the temperature range 300 ≤ T / K ≤ 650 K, is the highest reported for a germanium-free colusite to date. Our optimization strategy based on defect engineering in bulk materials is an exciting prospect for new low-cost thermoelectric systems.

We report a new class of cost-efficient n-type thermoelectric sulfides with a layered structure, namely MnBi4S7 and FeBi4S7. Theoretical calculations combined with synchrotron Xray/neutron diffraction analyses reveal the origin of their electronic and thermal properties. The complex low-symmetry monoclinic crystal structure generates an electronic band structure with a mixture of heavy and light bands near the conduction band edge, as well as vibrational properties favorable for high thermoelectric performance. The low thermal conductivity can be attributed to the complex layered crystal structure and to the existence of the lone pair of electrons in Bi 3+. This feature combined with the relatively high power factor lead to a figure of merit as high as 0.21 (700 K) in undoped MnBi4S7, making this material a promising n-type candidate for the low-and intermediate-temperature thermoelectric applications.