The Role of Zn in Chalcopyrite CuFeS2: Enhanced Thermoelectric Properties of Cu1-xZnxFeS2 with In Situ Nanoprecipitates

Chalcopyrite (CuFeS2) is a widespread natural mineral, composed of earth-abundant and nontoxic elements. It has been considered a promising n-type material for thermoelectric applications. In this work, a series of Zn-doped Cu1-xZnxFeS2 (x = 0-0.1) compounds are synthesized by vacuum melting combined with the plasma activated sintering process. The role of Zn in the chalcopyrite and its different effects on thermoelectric properties, depending on its concentration and location in the crystal lattice, are discussed. It is found that Zn is an effective donor which increases the carrier concentration and improves the thermoelectric properties of CuFeS2. When the content of Zn exceeds the solubility limit, Zn partially enters the Cu sites and forms in situ ZnS nanophase. This, in turn, shifts the balance between the anion and cation species which is re-established by the formation of antisite Fe/Cu defects. Beyond maintaining charge neutrality of the structure, such antisite defects relieve the lattice strain in the matrix and increase the solubility of Zn further. The highest ZT value of 0.26 is achieved at 630 K for Cu0.92Zn0.08FeS2, which represents an enhancement of about 80% over that of the pristine CuFeS2 sample.