Engineering Multiple Microstructural Defects for Record‐Breaking Thermoelectric Properties of Chalcopyrite Cu_1‐xAg_xGaTe₂

Abstract Defect engineering for vacancies, holes, nano precipitates, dislocations, and strain are efficient means of suppressing lattice thermal conductivity. Multiple microstructural defects are successfully designed in Cu_1‐xAg_xGaTe₂ (0 ≤ x ≤ 0.5) solid solutions through high‐ratio alloying and vibratory ball milling, to achieve ultra‐low thermal conductivity and record‐breaking thermoelectric performance. Extremely low total thermal conductivities of 1.28 W m⁻¹ K⁻¹ at 300 K and 0.40 W m⁻¹ K⁻¹ at 873 K for the Cu_0.5Ag_0.5GaTe₂ are observed, which are ≈79% and ≈58% lower than that of the CuGaTe₂ matrix. Multiple phonon scattering mechanisms are collectively responsible for the reduction of thermal conductivity in this work. On one hand, large amounts of nano precipitates and dislocations are formed via vibrating ball milling followed by the low‐temperature hot press, which can enhance phonon scattering. On the other hand, the difference in atomic sizes, distorted chemical bonds, elements fluctuation, and strained domains are caused by the high substitution ratio of Ag and also function as a center for the strong phonon scattering. As a result, the Cu_0.7Ag_0.3GaTe₂ exhibits a record high ZT_max of ≈1.73 at 873 K and ZT_ave of ≈0.69 between 300–873 K, which are the highest values of CuGaTe₂‐based thermoelectric materials.