Chalcogenides have been considered as promising thermoelectric materials because of their low cost, nontoxicity, and environmental benignity. In this work, we synthesized a series of CuSTe (0 ≤ x ≤ 1) alloys by a facile, rapid method of mechanical alloying combined with spark plasma sintering process. The CuSTe system provides an excellent vision of the competition between pure phase and phase transformation, entropy-driven solid solution, and enthalpy-driven phase separation. When the Te concentration increases, the CuSTe system changed from the pure monoclinic CuS at x = 0 to monoclinic CuSTe solid solution at 0.02 ≤ x ≤ 0.06 and then transforms to hexagonal CuSTe solid solution at 0.08 ≤ x ≤ 0.1. The phase separation of hexagonal CuTe in the hexagonal CuS matrix occurs at 0.3 ≤ x ≤ 0.7 and finally forms the hexagonal CuTe at x = 1. Owing to the changed band structure and the coexisted CuS and CuTe phases, greatly enhanced power factor was achieved in all CuSTe (0< x < 1) alloys. Meanwhile, the point defect introduced by the substitution of Te/S atoms strengthened the phonon scattering, resulting in a lowered lattice thermal conductivity in most of these solid solutions. As a consequence, CuSTe exhibits a maximum ZT value of 1.18 at 723 K, which is about 3.7 and 14.8 times as compared to the values of pristine CuS (0.32) and CuTe (0.08), respectively.

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