  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.
