Realizing Enhanced Thermoelectric Performance and Hardness in Icosahedral Cu₅FeS_4−xSe_x with High‐Density Twin Boundaries

Abstract Bornite (Cu₅FeS₄) is an Earth‐abundant, nontoxic thermoelectric material. Herein, twin engineering and Se alloying are combined in order to further improve its thermoelectric performance. Cu₅FeS_4−xSe_x (0 ≤ x ≤ 0.4) icosahedral nanoparticles, containing high‐density twin boundaries, have been synthesized by a colloidal method. Spark plasma sintering retains twin boundaries in the pellets sintered from Cu₅FeS_4−xSe_x colloidal powders. Thermoelectric property measurement demonstrates that alloying Se increases the carrier concentration, leading to much‐improved power factor in Se‐substituted Cu₅FeS₄, for example, 0.84 mW m⁻¹ K⁻² at 726 K for Cu₅FeS_3.6Se_0.4; low lattice thermal conductivity is also achieved, due to intrinsic structural complexity, distorted crystal structure, and existing twin boundaries and point defects. As a result, a maximum zT of 0.75 is attained for Cu₅FeS_3.6Se_0.4 at 726 K, which is about 23% higher than that of Cu₅FeS₄ and compares favorably to that of reported Cu₅FeS₄‐based materials. In addition, the Cu₅FeS_4−xSe_x samples containing twin boundaries also obtain improved hardness compared to the ones fabricated by melting‐annealing or ball milling. This work demonstrates an effective twin engineering‐composition tuning strategy toward enhanced thermoelectric and mechanical properties of Cu₅FeS₄‐based materials.