Crystal Structure–Dependent Defect Architectures in (Ag, Mg)‐Doped Cu ₂ SnS ₃ to Achieve Low Thermal Conductivity

As an environment‐friendly and cost‐effective thermoelectric (TE) material, Cu 2 SnS 3 (CTS) has attracted considerable research interest aimed at improving its TE performance through crystal phase transformation and defect engineering. However, the underlying microscopic mechanisms governing defect formation and their correlation with crystal structure remain insufficiently explored. In this study, a series of (Ag, Mg, Al)‐doped CTS samples were synthesized to systematically investigate the relationship between crystal structure and defect characteristics. Notably, nanoscale elemental enrichment regions were observed within grains of monoclinic symmetry in Cu 1.9 Ag 0.1 Sn 0.9 Mg 0.1 S 3 (CTSS10M10), whereas wave‐like defects and dislocation arrays resembling gill structures were identified in tetragonal‐symmetry grains. More importantly, defect‐induced stress in the tetragonal phase was traced primarily to specific (0 0 2) lattice planes. The local coordination environment of Cu atoms, which is closely associated with structural disorder, was also examined. Ultimately, CTSS10M10 achieved a peak zT of 0.28 at 750 K, attributable to improved electrical properties via hole doping and reduced lattice thermal conductivity through enhanced phonon scattering. This work provides new insights into optimizing thermoelectric performance through crystal structure regulation and defect control.