Comment on the "Discovery of high-performance thermoelectric copper chalcogenide using modified diffusion-couple high-throughput synthesis and automated histogram analysis technique" by T. Deng, T. Xing, M. K. Brod, Y. Sheng, P. Qiu, I. Veremchuk, Q. Song, T.-R. Wei, J. Yang, G. J. Snyder, Y. Grin, L. Chen and X. Shi, Energy Environ. Sci., 2020, 13, 3041

The recent paper by Deng et al. showed that a copper sulphide with Cu7Sn3S10 composition is a promising thermoelectric material reaching a figure of merit ZT of 0.8 at 750 K via Cl doping. However, structural reinvestigations of the Cu7Sn3S10 sample (i.e., Cu2.10Sn0.90S3) prepared in the reported conditions show that the phase of interest is not tetragonal (space group I (4) over bar 2m, a(t) = 5.4164 angstrom, c(t) = 10.832 angstrom), but crystallizes in the cubic symmetry (space group P (4) over bar 3n, a(c) = 10.832 angstrom) with a composition close to Cu22Sn10S32 similar to the results published by Pavan Kumar et al. These observations indicate that the main phase around the composition Cu7Sn3S10 is cubic, and a revision needs to be imposed for the understanding of the relationships between the structure and thermoelectric properties in this family.