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

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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 AgBiSe2-2xSxTex 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 AgBiSe2-2xSxTex 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 AgBiSe2-2xSxTex 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 AgBiSe2-2xSxTex.