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

The ternary diamond-like compound Cu 2 SnSe 3 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 Cu 2 SnSe 3 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 Cu 2 SnSe 3 . Owing to coexisting phases and the order–disorder transition in the orthorhombic phase, the Ag-doped Cu 2 SnSe 3 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 Cu 2 Sn 0.93 Ag 0.07 Se 3 is double that of undoped Cu 2 SnSe 3 .