Characterization of Electronic Transport Properties of Narrow-Band Gap Fe(Se_1-xTe_x)₂ Alloys via the Two-Band Model

Environmentally sustainable thermoelectric technologies can be more broadly applied in industries once the performance of thermoelectric materials is improved. Several approaches have been proposed to improve the electronic transport properties of thermoelectric materials. The effects of each approach on the electronic properties can be evaluated by changes in the band parameters. The Single Parabolic Band (SPB) model has been widely used to determine the effect of different materials engineering strategies on band parameters, such as the density-of-states effective mass and deformation potential. However, when the material has a narrow band gap, the Two-Band (TB) model better describes the changes in band parameters, as it includes the bipolar conduction from the minority carrier band. Here, the band parameters of previously reported Fe(Se_1-xTe_x)₂ (x = 0, 0.2, 0.6, 0.8, 1) alloys, whose band gap significantly decreases with x, have been estimated using the SPB and TB models. While the x-dependent band parameters obtained via the SPB model varied abruptly with x, all the band parameters estimated by the TB model changed linearly with x. The abruptness observed in the band parameters of the SPB model can be attributed to artifacts reflected in the single band, which occurs when the minority carrier band and band gap change are not included. The bipolar thermal conductivity of Fe(Se_1-xTe_x)₂ alloys was also calculated using the TB model, and is understood in terms of changes in the weighted mobility ratio, Hall carrier concentration, and band gap in terms of alloy composition, x.