Ultra-low thermal conductivity and improved thermoelectric performance in disordered nanostructured copper tin sulphide (Cu2SnS3, CTS)

Copper tin sulphide (Cu2SnS3, CTS) is a promising p-type thermoelectric material. In the present work, we have investigated the cation disorder in CTS disks made by sintering powders produced via high energy reactive ball-milling. The crystalline structures, electronic and thermal properties were systematically investigated. As-milled CTS shows a disordered cubic structure (c-CTS), preserved with thermal treatment up to 500 C. By increasing the thermal treatment temperature, CTS gradually evolves towards the ordered monoclinic structure (m-CTS), reaching complete order at 650 C. The disordered CTS has several times higher zT than the ordered CTS. In fact, ordered CTS has high thermopower, up to 700 mV/K, and high electrical resistivity leading to zT < 0.05 above 700K; whereas, disordered (c-CTS) has comparatively high zT~0.30 above 700K. This has been related to the lower electrical resistivity, and the ultra-low thermal conductivity (k~0.26 W/m-K), resulting from the disordered structure, promoting Phonon-Glass-Electron-Crystal (PGEC) characteristics. In our best knowledge, zT~0.30 above 700K is the highest in CTS samples without acting on the chemistry of the system.