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

Silver bismuth diselenide (AgBiSe2) has much attention as an efficient thermoelectric material due to its low thermal conductivity. However, AgBiSe2 exhibits multiple crystal structural transitions with temperature, and high thermoelectric performance was realized only in high-temperature cubic phase. We previously reported the stabilization of cubic phase in AgBiSe2-2xSxTex with x = 0.6-0.8 at room temperature by high-entropy-alloy (HEA) approach. The cubic HE-type AgBiSe0.8S0.6Te0.6 achieved a high ZT value of 0.8 at 748 K. In this paper, we succeeded in stabilizing the cubic phase in AgBiSe2-2xSxTex with x = 0-0.6 by ice-quenching method, and investigated the HE effect on the thermoelectric properties. Cubic AgBiSe2-2xSxTex exhibited n-type conductivity from 300 K to 10 K. We found that electronic conductivity was largely increased around room temperature with increasing the amount of S and Te, although carrier concentration showed almost the same values. The S and Te substitutions induced the variation of band structure, resulting in the carrier mobility enhancement. Furthermore, thermal conducutivity showed reduction tendency with increasing the amount of S and Te due to enhancement of phonon scattering. Simultaneous electronic conductivity increase and thermal conductivity reduction resulted in the systematic improvement of the ZT values for HE-type cubic AgBiSe2-2xSxTex.