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

Copper vacancies (VCu) 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 VCu and InCu anti-site defects over a wide concentration range. The InCu 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 VCu 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.