Investigations on electrical and thermal transport properties of Cu₂SnSe₃ with unusual coexisting nanophases

The ternary diamond-like compound Cu₂SnSe₃ is a potential thermoelectric material. Its Cu-Se conducting network leaves Sn as a likely doping site to optimize the carrier concentration without much deterioration to the carrier mobility. Thus, the precise determination on the intricate phase structures of Cu₂SnSe₃ is critical. Here, we first use the atomically resolved scanning transmission electron microscopy and reveal an unusual phase coexistence (monoclinic and orthorhombic phases) in Ag-doped Cu₂SnSe₃. Owing to coexisting phases and the order–disorder transition in the orthorhombic phase, the Ag-doped Cu₂SnSe₃ shows an unusual three-stage behavior in its temperature-dependent electrical transport properties and achieves record high power factors in this system. The observed three-stage behavior due to the phase coexistence is supported by the first principle calculations, Hall measurements, and optical diffuse reflectance measurements. To fully understand the band structures and the coherent interface between these coexisting phases, a band contact model is proposed that could well explain the three-stage electrical transport behavior. Moreover, the phase coexistence is observed at the nanoscale regime, thus providing a high density of phase boundaries. Such coexisting nanophases play an important role in lowering the lattice thermal conductivity. As a result, the ZT value obtained in Cu₂Sn_0.93Ag_0.07Se₃ is double that of undoped Cu₂SnSe₃.