Hierarchically structured thermoelectric materials in quaternary system cu-zn-sn-s featuring a mosaic-type nanostructure

Multinary chalcogenide semiconductors in the Cu-Zn-Sn-S system have numerous potential applications in the fields of energy production, photocatalysis and nonlinear optics, but characterization and control of their microstructures remains a challenge because of the complexity resulting from the many mutually soluble metallic elements. Here, using state-of-the-art scanning transmission electron microscopy, energy dispersive spectroscopy, first-principles calculations and classical molecular dynamics simulations, we characterize the structures of promising thermoelectric materials Cu₂(Zn,Sn)S₃ at different length scales to gain a better understanding of how the various components influence the thermoelectric behavior. We report the discovery of a mosaic-type domain nanostructure in the matrix grains comprising well-defined cation-disordered domains (the "tesserae") coherently bonded to a surrounding network phase with semiordered cations. The network phase is found to have composition Cu_4+xZn_xSn₂S₇, a previously unknown phase in the Cu-Zn-Sn-S system, while the tesserae have compositions closer to that of the nominal composition. This nanostructure represents a new kind of phonon-glass electron-crystal, the cation-disordered tesserae and the abrupt domain walls damping the thermal conductivity while the cation-(semi)ordered network phase supports a high electronic conductivity. Optimization of the hierarchical architecture of these materials represents a new strategy for designing environmentally benign, low-cost thermoelectrics with high figures of merit.