Synthesis and property evaluation of CuFeS2−x as earth-abundant and environmentally-friendly thermoelectric materials

► CuFeS2 polycrystals were synthesized by mechanical alloying and spark plasma sintering. ► Thermoelectric performance of CuFeS2−x alloys were investigated as a function of S content. ► The composition CuFeS1.8 shows a high ZT value of 0.21 at 573 K. ► S-deficiency can reduce electrical resistivity and thermal conductivity of CuFeS2−x alloys. ► A tetragonal to cubic phase transition was observed in CuFeS2−x alloys.