Thermoelectric properties of zinc-doped Cu₅Sn₂Se₇ and Cu₅Sn₂Te₇

High-performance thermoelectric materials are currently being sought after to recycle waste heat. Copper chalcogenides in general are materials of great interest because of their naturally low thermal conductivity and readily modifiable electronic properties. The compounds Cu5Sn2Q7 were previously reported to have metal-like properties, which is not a desirable characteristic for thermoelectric materials. The aim of this study was to reduce the carrier concentration of these materials by Zn-doping, and then investigate the electronic and thermoelectric properties of the doped materials in comparison to the undoped ones. The compounds were synthesized using both the traditional solid-state tube method and ball-milling. The crystal structures were characterized using powder X-ray diffraction, which confirmed that all materials crystallize in the monoclinic system with the space group C2. With the partial substitution of zinc for copper atoms, the compounds exhibited an overall improvement in their thermoelectric properties. Figure of merit values were determined to be 0.20 for Cu4ZnSn2Se7 at 615 K and 0.05 for Cu4ZnSn2Te7 at 575 K.