Defect-Induced Local Symmetry Breaking Triggers Lattice Softening for High Thermoelectric Performance in CuInTe₂...

Copper vacancies (V_Cu) are crucial for optimizing the thermoelectric performance of CuInTe₂-based compounds by governing the hole concentration and electrical transport. However, their limited solubility and restricted ability to suppress lattice thermal conductivity hinder further improvements. In this work, we have synthesized a series of single-phase (Cu₂Te)_{1- x}(In₂Te₃)_x samples based on the Cu₂Te-In₂Te₃ pseudo-binary phase diagram, which enabled the regulation of the density of V_Cu and In_Cu anti-site defects over a wide concentration range. The In_Cu anti-site defect disrupts the local lattice symmetry, driving cations away from the tetrahedral centers and softening the lattice, thereby reducing the room-temperature lattice thermal conductivity from 5.9 W m^-1 K^-1 in the intrinsic sample to 1.81 W m^-1 K^-1 in (Cu₂Te)_0.4(In₂Te₃)_0.6. Meanwhile, the increased concentration of V_Cu leads to the broadening of impurity levels, raising the room-temperature carrier concentration from 1.0 × 10¹⁸ cm^-3 in intrinsic CuInTe₂ to 5.7 × 10¹⁹ cm^-3 in (Cu₂Te)_0.49(In₂Te₃)_0.51. As a result of the synergistic optimization of electrical and thermal transport properties, the (Cu₂Te)_0.49(In₂Te₃)_0.51 sample achieved a maximum zT of 1.21 at 873 K. This work demonstrates that pseudo-binary solid solution is an effective approach to improving the electrical and thermal properties of chalcopyrite compounds, providing a new pathway for achieving higher thermoelectric performance.