Phase diagram of ternary Cu-Ga-Te system and thermoelectric properties of chalcopyrite CuGaTe₂ materials

Among all types of thermoelectric materials, chalcopyrite CuGaTe₂ has been viewed as a promising candidate for use of thermoelectric generator due to its high figure-of-merit (zT) at the high temperatures. Herein the 923 K isothermal section of ternary Cu-Ga-Te system is determined, using various thermally-equilibrated Cu-Ga-Te alloys, and ternary CuGaTe₂ phase is stabilized within the compositional region of 48.0–53.0 at%Te and 25.0 at%-30.0 at%Cu. Moreover, the solubility of Cu in binary Ga₂Te₃ and Ga₃Te₄ compounds at 923 K is negligible, while that in GaTe phase reaches 7.9 at%Cu. The as-determined isothermal section, depicting the phase stability regime of CuGaTe₂, provides options for precisely locating the compositions of CuGaTe₂-based materials that lead to promising and reproducible thermoelectric properties. A zT peak of 0.6 has been achieved in the Bridgman-grown Cu₂₅Ga₂₆Te₄₉ alloy at 750 K, which is nearly eight times higher than the neighboring Cu₂₈Ga₂₅Te₄₇ alloy, presumably due to the fact that the Cu₂₅Ga₂₆Te₄₉ alloy, which exhibits high phase purity of CuGaTe₂, has lower lattice thermal conductivity (κ_L∼1.3 (W/mK) and higher power factor (PF∼11.2 (μW/mK²)), comparing with that of Cu₂₈Ga₂₅Te₄₇ alloy (κ_L∼1.8 (W/mK) and PF∼1.9 (μW/mK²)), which locates in a three-phased CuGaTe₂+Cu₂Te + Cu₉Ga₄ region, with only a slight deviation in the starting composition.