Talnakhite: A potential n-type thermoelectric sulphide with low thermal conductivity

The mineral talnakhite, Cu18Fe16S32, is an n-type semiconductor with low thermal conductivity (average value of 1.5 W m(-1) K-1), making it an attractive candidate for thermoelectric applications. The effect of partial cation substitutions and of deviations from the ideal Cu:Fe ratio on the thermoelectric properties of this material, has been investigated through synthesis of Cu17.58M0.02Fe17.6S32 (M = Ag, In, Zn) and Cu17.6+xFe17.6-xS32 (-0.03 <= x <= 0.03) by high-temperature methods. The results demonstrate that talnakhite exhibits a narrow range of compositional stability for substitution at the cation sites. X-ray photoelectron spectroscopy (XPS) measurements indicate that all compositions contain Fe3+ and Fe2+ cations, together with Cu+. The electrical and thermal transport properties show two anomalies, at approximately 460 and 510 K, which can be related to structural phase transitions. The maximum value of the thermoelectric figure of merit occurs at the temperature of the first structural phase transition, making talnakhite a potential n-type candidate for near room-temperature thermoelectric applications. While substitution with silver, zinc or indium does not lead to any significant improvement in thermoelectric performance, changes in the Cu:Fe ratio result in significant reductions in the total thermal conductivity. This is likely to be associated with increased point defect scattering due to the presence of additional vacancies at the cation sites over which iron and copper are partially ordered. For copper-poor phases, the combination of a slightly improved power factor with a reduced thermal conductivity results in an increase in the figure-of-merit by approximately 20% when compared to the stoichiometric phase.

Automated summary

The effect of partial cation substitutions and deviations from the ideal Cu:Fe ratio on the thermoelectric properties of talnakhite has been investigated. The results show that talnakhite has a narrow range of compositional stability. The copper-poor phases exhibit slightly higher thermoelectric coefficients and reduced thermal conductivity, making them potential candidates for near room-temperature thermoelectric applications.