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.

To investigate pathways to adjust the charge carrier concentration and optimize the thermoelectric properties, we characterized structural properties, thermal stability, and thermoelectric performance of pristine and Cl-doped Cu5+εSn2−εS7. We demonstrate that Cl doping in Cu5Sn2S7-type monoclinic compounds induces a collapse of the long-range cationic ordering, ultimately leading to a sphalerite-type cubic phase characterized by ordered [Sn­(S,Cl)4] x clusters. The change in crystal structure symmetry upon Cl doping is analyzed by Rietveld refinements against X-ray powder diffraction data, transmission electron microscopy, Mössbauer and X-ray absorption spectroscopy, and low- and high-temperature transport property measurements. The thermoelectric properties of the so-obtained cubic sphalerite Cu5+εSn2−εS7–y Cl y (0 ≤ ε ≤ 0.133, y = 0.35, 0.70) are strongly enhanced with respect to the undoped Cu5Sn2S7: the power factor improves slightly while both electronic and lattice contributions to the thermal conductivity are reduced. Overall, single-phase Cl-doped Cu5.133Sn1.866S7–y Cl y (y = 0.35, 0.70) compounds exhibit high thermoelectric performance, reaching a maximum ZT of 0.45 at 670 K.