Phonon Scattering and Thermal Transport in PbSe and Medium Entropy Thermoelectric PbSe_0.5Te_0.25S_0.25 with Defects of Different Dimensions

Defect engineering has been proven to be effective in optimizing the thermoelectric performance by tailoring the lattice thermal conductivity. Understanding the roles of defects with different dimensions on heat-carrying phonons is crucial. In this study, we investigated the lattice dynamics and thermal transport properties of PbSe and medium entropy PbSe_0.5Te_0.25S_0.25 with different defects, using a machine learning interatomic potential. We find that the introduction of 3% Pb vacancies reduces lattice thermal conductivity (<i>k</i>_<i>L</i>) of PbSe by approximately 40%, similar to dislocations (with a density of 2 × 10¹⁶ m^-2) and nanograins (grain size of 4000 nm³). Vacancies enhance phonon scattering at frequencies above 0.5 THz, whereas dislocations, stacking faults, and grain boundaries primarily hinder phonon propagation below 1.5 THz. In contrast, in PbSe_0.5Te_0.25S_0.25, vacancies have weaker effects on thermal transport suppression due to the intrinsic entropy-induced high-frequency phonon scattering; higher-dimensional defects, such as grain boundaries, stacking faults, and dislocations, are more effective for reducing <i>k</i>_<i>L</i> by enhancing phonon scattering at frequencies lower than 1 THz. This study reveals the mechanisms by which defects influence lattice thermal conductivity and provides valuable guidance for optimizing the thermal transport in entropy engineered systems.