Structural Evolution of High‐Performance Mn‐Alloyed Thermoelectric Materials: A Case Study of SnTe

Mn alloying in thermoelectrics is a long‐standing strategy for enhancing their figure‐of‐merit through optimizing electronic transport properties by band convergence, valley perturbation, or spin‐orbital coupling. By contrast, mechanisms by which Mn contributes to suppressing thermal transports, namely thermal conductivity, is still ambiguous. A few precedent studies indicate that Mn introduces a series of hierarchical defects from the nano‐ to meso‐scale, leading to effective phonon scattering scoping a wide frequency spectrum. Due to insufficient insights at the atomic level, the theory remains as phenomenological and cannot be used to quantitatively predict the thermal conductivity of Mn‐alloyed thermoelectrics. Herein, by choosing the SnTe as a case study, aberration‐corrected transmission electron microscopy (TEM)/scanning transmission electron microscopy (STEM) to characterize the lattice complexity of Sn 1.02− x Mn x Te is employed. Mn as a “dynamic” dopant that plays an important role in SnTe with respect to different alloying levels or post treatments is revealed. The results indicate that Mn precipitates at x = 0.08 prior to reaching solubility (≈10 mol%), and then splits into Mn Sn substitution and γ‐MnTe hetero‐phases via mechanical alloying. Understanding such unique crystallography evolution, combined with a modified Debye‐Callaway model, is critical in explaining the decreased thermal conductivity of Sn 1.02− x Mn x Te with rational phonon scattering pathways, which should be applicable for other thermoelectric systems.