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.