In Situ Observation of Dynamic Defect Evolution and Phase Transition in Mn‐Doped Sb 2 Te 3

ABSTRACT Defect engineering has long been a primary strategy for optimizing thermoelectric materials. Understanding the dynamic behavior of atomic defects under real working conditions is crucial for revealing structure‐property relationships. Here, we combine in situ atomic‐resolution scanning transmission electron microscopy (STEM) with density functional theory (DFT) to unravel the dynamic evolution of manganese (Mn) dopants in the p‐type mid‐temperature TE material antimony telluride (Sb 2 Te 3 ). Initially, Mn forms both interstitial defects and Mn Sb substitutional defects. Upon heating, thermally activated Mn diffusion leads to Mn‐Sb mixed regions, degraded interfacial coherency, and a structural α‐to‐δ phase transition. This dynamic structural evolution contributes to a suppression in lattice thermal conductivity and modest ZT enhancement at high temperatures. Our work directly links dynamic structural evolution to transport properties, offering new insights into defect engineering that could be synergistically combined with other optimization methods to design advanced thermoelectric materials.