  As thermoelectric generators can convert waste heat into electricity, they
play an important role in energy harvesting. The metal chalcogenide AgBiSe2 is
one of the high-performance thermoelectric materials with low lattice thermal
conductivity (klat), but it exhibits temperature-dependent crystal structural
transitions from hexagonal to rhombohedral, and finally a cubic phase as the
temperature rises. The high figure-of-merit ZT is obtained only for the
high-temperature cubic phase. In this study, we utilized the high-entropy-alloy
(HEA) concept for AgBiSe2 to stabilize the cubic phase throughout the entire
temperature range with enhanced thermoelectric performance. We synthesized
high-entropy-type AgBiSe2-2xSxTex bulk polycrystals and realized the
stabilization of the cubic phase from room temperature to 800 K for x > 0.6.
The ultra-low klat at of 0.30 Wm^-1K^-1 and the high peak ZT 0.9 at around 750
K were realized for cubic AgBiSe2-2xSxTex without carrier tuning. In addition,
the average ZT value of x = 0.6 and 0.7 for the temperature range of 360-750 K
increased to 0.38 and 0.40, respectively, which are comparable to the highest
previously reported values.
