Revealing an elusive metastable wurtzite CuFeS2 and the phase switching between wurtzite and chalcopyrite for thermoelectric thin films

As the alternative for the high-performance thermoelectric semiconductors such as GeTe, PbTe, and other compounds with rare and expensive elements, the cost-effective and abundant sulfides have been considered as one of the promising materials and garnered intensive interest. In this study, a rarely reported metastable wurtzite-type phase of CuFeS2 has been successfully obtained as thin film grown on quartz substrates via magnetron sputtering. Subsequent temperature-dependent depositions were performed to establish a phase transition between the metastable wurtzite and the chalcopyrite phase in the temperature range between room temperature and 473 K while maintaining the nominal composition ratio of CuFeS2. The wurtzite metastable analogue can be recovered and stabilized when the temperature reached 573 K and above. Thermoelectric properties were evaluated and constituted the first experimental result of the inorganic CuFeS2 thin film for thermoelectrics. The phase change between the wurtzite and chalcopyrite structure induced a significant effect on the thermoelectric properties associated with the lattice disorder and carrier scattering. The reduced thermal conductivity compared with bulk was demonstrated by the picosecond time-domain thermoreflectance method, which was found to be well correlated with the polycrystal sizes. The work provides new insight into synthesizing and manipulating the metastable wurtzite phase of the ternary I-III-VI for future applications in thermoelectrics, solar cells, and other elec-tronic applications.(C) 2022 The Author(s). Published by Elsevier Ltd on behalf of Acta Materialia Inc.

Automated summary

In this study, a rarely reported metastable wurtzite-type phase of CuFeS2 thin film was successfully obtained and a phase transition between the metastable wurtzite and the chalcopyrite phase was observed. The phase change significantly affected the thermoelectric properties. It was found that the wurtzite phase could be recovered and stabilized between room temperature and 473 K. Additionally, the reduced thermal conductivity of the thin film compared with bulk was demonstrated, which was correlated with the polycrystal sizes.