Enargite Cu₃PS₄: A Cu–S-Based Thermoelectric Material with a Wurtzite-Derivative Structure

Compound semiconductors derived from ZnS (zincblende and wurtzite) with tetrahedral framework structures have functions for various applications. Examples of such materials include Cu–S-based materials with zincblende-derivative structures, which have attracted attention as thermoelectric (TE) materials over the past decade. This study illuminates superior TE performance in polycrystalline samples of enargite Cu₃P_{1− x}Ge_xS₄ with a wurtzite-derivative structure. The substitution of Ge for P dopes holes into the top of the valence band composed of Cu-3d and S-3p, whereby its multiband characteristic leads to a high TE power factor. Furthermore, a reduction in the grain size to 50–300 nm can effectively decrease phonon mean free paths, leading to low thermal conductivity. These features result in a dimensionless TE figure of merit ZT of 0.5 at 673 K for the x = 0.2 sample. Environmentally benign and low-cost characteristics of the constituent elements of Cu₃PS₄, as well as its high-performance thermoelectricity, make it a promising candidate for large-scale TE applications. Furthermore, this finding extends the development field of Cu–S-based TE materials to those with wurtzite-derivative structures.