High entropy effect on thermoelectric properties of the nonequilibrium cubic phase of AgBiSe2−2xSxTex with x = 0–0.6

Abstract

Silver bismuth diselenide (AgBiSe₂) has attracted much attention as an efficient thermoelectric material owing to its low thermal conductivity. However, AgBiSe₂ exhibits multiple crystal structural transitions with temperature, and high thermoelectric performance was realized only in the high-temperature cubic phase. We previously reported the stabilization of the cubic phase in AgBiSe_2−2xS_xTe_x with x = 0.6–0.8 at room temperature using a high-entropy-alloy (HEA) approach. In this paper, we succeeded in stabilizing the cubic phase in AgBiSe_2−2xS_xTe_x with x = 0–0.6 using an ice-quenching method and investigated the HE effect on thermoelectric properties below room temperature to avoid the emergence of a hexagonal phase above room temperature. Cubic AgBiSe_2−2xS_xTe_x exhibited n-type conductivity at 10–300 K. The increase in electrical conductivity is due to a combined effect of increased charge density and electron mobility, depending on the sample. It is evident that charge carrier density strongly increases in the x = 0.4 sample while the mobility remains roughly the same compared to the x = 0.0 sample. In x = 0.2 and 0.6 samples, electronic conductivity increases primarily due to enhanced mobility. S and Te substitutions induced a variation in the band structure, resulting in carrier mobility enhancement. Furthermore, thermal conductivity showed reduction tendency with increasing amounts of S and Te due to the enhancement of phonon scattering. Simultaneous electronic conductivity increase and thermal conductivity reduction resulted in the systematic improvement of ZT values for HE-type cubic AgBiSe_2−2xS_xTe_x.