Asymmetric Homogeneity and Transport Properties of Isotropic Copper-Based Ternary Chalcogenides

This research delves into the copper-based chalcogenides, unveiling the synthesis and characterization of two stable ternary copper chalcogenides, Cu₅Se₇Te₃ and Cu₆SeTe₃, at 523 K. An isothermal section of Cu-Se-Te constructed through the detailed analysis upon the postannealed alloys has brought a comprehensive understanding to the homogeneity of these compounds. The Cu₆SeTe₃, located near the Cu₂(Se, Te) solid solution, showcases an asymmetric compositional region extending from chemically stoichiometric Cu₆Se₂Te₂ to off-stoichiometric Cu₆SeTe₃. Structure refinement from the single-crystal X-ray diffraction has confirmed the cubic symmetry of Cu₆SeTe₃, yet the Te atoms deviate from the expected position. In the cubic lattice composed of orderly arranged selenium and tellurium, copper ions behave like a liquid at high temperatures, which is characteristic of phonon-liquid electron-crystal (PLEC) chalcogenides. Cu₆SeTe₃ and Cu₆Se₂Te₂ show no phase transitions between 100 and 600 K, signifying their thermal stability in practical use. Moreover, Cu₆Se₂Te₂ reached an ideal carrier concentration of 6 × 10¹⁹ cm^-3, alongside an enhancement in the power factor and a decrease in thermal conductivity than the Te-rich Cu₆SeTe₃. These composition adjustments in the Cu₆SeTe₃ phase also raise the Z value.