Crossover from Germanite to Renierite-Type Structures in Cu_22–xZn_xFe₈Ge₄S₃₂ Thermoelectric Sulfides

Copper-rich sulfides with the composition Cu22–xZnxFe8Ge4S32 (0 ≤ x ≤ 2) have been investigated. The detailed structural study combining powder X-ray diffraction, electron microscopy, and Mössbauer spectroscopy shows a crossover from a germanite-type domain for 0 ≤ x ≤ 0.4 to a renierite-type domain for 1.2 ≤ x ≤ 2.0, with a biphasic region for x = 0.8. Both sulfides are p-type thermoelectrics due to the presence of the conductive Cu–S network containing univalent copper as a major cationic element. All these sulfides exhibit a semiconducting behavior. However, the renierite-type sulfides show a higher resistivity and a lower thermal conductivity than the germanite ones. As a consequence, the figures of merit ZT are similar for germanite and renierite domains, reaching values of around 0.23 at 700 K.