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+<i>x</i>Zn_<i>x</i>Sn₂S₇, 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