Long-Range Cationic Order Collapse Triggered by S/Cl Mixed-Anion Occupancy Yields Enhanced Thermoelectric Properties in Cu₅Sn₂S₇

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