Cu–S-based thermoelectric compounds with a sphalerite-derived disordered crystal structure

Low lattice thermal conductivity is observed in compounds having disordered atomic configurations in their crystal structures. At temperatures above 300 ​K, a Cu–S-based semiconducting compound, Cu3.1Nb0.25Sn0.9S4, with a sphalerite-derived disordered structure, exhibits thermal conductivity less than 1 ​W ​K−1 ​m−1. In this study, we synthesized and tested the thermoelectric properties of the hole-doped derivatives, Cu3.1Nb0.25−xTixSn0.9S4 (x ​= ​0, 0.125, 0.25) and Cu3.1Ti0.1875Sn0.9S4, up to 673 ​K. The hole-carrier doping decreased the electrical resistivity, improving the power factor. Cu3.1Ti0.25Sn0.9S4 shows a dimensionless figure of merit ZT of 0.1/0.6 ​at 300/673 ​K due to its inherent low lattice thermal conductivity and improved power factor. The thermoelectric properties of the Cu3.1Nb0.25Sn0.9S4-based compounds were compared with those of Cu3SbS4-based compounds with an ordered arrangement of Cu atoms in a similar sphalerite-derived structure, showing that the disordered atomic arrangement is responsible for the enhanced scatterings of both phonons and electrons.