Design of Domain Structure and Realization of Ultralow Thermal Conductivity for Record‐High Thermoelectric Performance in Chalcopyrite

Abstract Chalcopyrite compound CuGaTe₂ is the focus of much research interest due to its high power factor. However, its high intrinsic lattice thermal conductivity seriously impedes the promotion of its thermoelectric performance. Here, it is shown that through alloying of isoelectronic elements In and Ag in CuGaTe₂, a quinary alloy compound system Cu_1−xAg_xGa_0.4In_0.6Te₂ (0 ≤ x ≤ 0.4) with complex nanosized strain domain structure is prepared. Due to strong phonon scattering mainly by this domain structure, thermal conductivity (at 300 K) drops from 6.1 W m⁻¹ K⁻¹ for the host compound to 1.5 W m⁻¹ K⁻¹ for the sample with x = 0.4. As a result, the optimized chalcopyrite sample Cu_0.7Ag_0.3Ga_0.4In_0.6Te₂ presents an outstanding performance, with record‐high figure of merit (ZT) reaching 1.64 (at 873 K) and average ZT reaching 0.73 (between ≈300 and 873 K), which are ≈37 and ≈35% larger than the corresponding values for pristine CuGaTe₂, respectively, demonstrating that such domain structure arising from isoelectronic multielement alloying in chalcopyrite compound can effectively suppress its thermal conductivity and elevate its thermoelectric performance remarkably.