Atomistic Phase Transition Mechanism and Its Relationship with the High Thermoelectric Performance in SnSe

SnSe maintains a high figure of merit (ZT) at elevated temperatures, which is closely linked to its atomic structure. However, the nature of its high‐temperature (HT) phase remains controversial, primarily due to the lack of direct observations for thermal‐induced structural change. Here, the layer‐shuffling phase transition mechanism is characterized at the atomic scale by in situ transmission electron microscopy (TEM), assisted by theoretical calculations and machine learning. Notably, a slightly distorted HT α phase dominates at elevated temperatures, likely induced by the breakdown of long‐range coherence because of the dynamic disorder of Sn atoms. Further investigations into the electrical and thermal transport properties indicate that the HT α phase primarily contributes to the excellent thermoelectric performance of SnSe reported previously. This study provides a fundamental understanding of structure‐thermoelectric property correlations, paving the way for the development of advanced thermoelectric materials.