Nature of Thermal Hysteresis of Thermoelectric Properties in Ag₂Te_xS_1–x Compounds

Ag 2 Te x S 1– x usually undergo various phase structures upon heating or cooling processes; however, the correlation between the heat treatment, the phase structure, and the physical properties is still a controversy. Herein, three different phases are realized for Ag 2 Te x S 1– x (0.35 ≤ x ≤ 0.65) samples during the heat treatment, including the low-temperature crystalline phase, amorphous phase, and high-temperature cubic phase. The metastable amorphous phase is an intermediate phase formed during transition from the high-temperature cubic phase to the low-temperature crystalline phase upon cooling via a solid-state conversion rather than the conventional liquid quenching process. The relative content of these three phases is highly sensitive to the heat treatment process. This as-formed low-temperature crystalline phase, amorphous phase, and high-temperature cubic phase convert into the low-temperature crystalline phase and high-temperature cubic phase through long-time dwelling at the temperature below or above the transition temperature around 567 K, respectively. The status of the low-temperature crystalline phase, amorphous phase, and high-temperature cubic phase significantly affects the thermoelectric properties, resulting in the thermal hysteresis of thermoelectric properties. Below the phase transition temperature ( T M ), the electrical conductivity of the amorphous phase surpasses that of the low-temperature crystalline phase, which shows a growth of 112% for the Ag 2 Te 0.60 S 0.40 sample annealed at 823 K in comparison with that of the sample annealed at 473 K. For Ag 2 Te 0.50 S 0.50 samples annealed at 473 K, the maximum ZT value reaches 1.02 at 623 K during the initial test, while the maximum ZT value is improved to 1.34 at 523 K in the second-round test.