Microstructure of Cu₂S nanoprecipitates and its effect on electrical and thermal properties in thermoelectric Cu₂Zn_0.2Sn_0.8S₃ ceramics

The microstructures of Cu₂Zn_0.2Sn_0.8S₃ ceramics with high electrical conductivity and low thermal conductivity were investigated by a combination of selected area electron diffraction, high-resolution transmission electron microscopy, X-ray energy dispersive spectroscopy and atom force microscopy techniques. The plate-like tetragonal metastable Cu₂S nanoprecipitates with elongated and equiaxed shape were embedded in a distinctive mosaic nanostructure with roughly 10 nm wide facetted domains (fully disordered phase) surrounded by a ∼5 nm wide connective phase (a semi-ordered monoclinic-Cu₄ZnSn₂S₇ phase) were observed. These metastable Cu₂S nanoprecipitates show clear orientation relationships with the matrix that the plates align with three crystal axes of cubic lattice. A combination of conductive atomic force microscopy and Kelvin probe force microscopy reveals that the nanoprecipitates have higher electrical conductivity than the matrix due to the higher carrier density, which can inject into the matrix and enhance the total electric conductivity of the sample. Furthermore, a mechanism of phonon scattering is proposed based on the effects of the occupation disorder of Cu atoms in Cu₂S nanoprecipitates, coherent heterointerfaces between Cu₂S and matrix, and the extended strain field in the matrix regions adjacent to the Cu₂S nanoprecipitates.