Vacancy-induced phonon localization and lattice softening for reduced thermal conductivity in Mg₃Sb₂

With the natural abundance, low cost, and compatibility with sustainable technologies, Mg₃Sb₂ has emerged as a promising mid-temperature thermoelectric material. Intrinsic point defects, particularly vacancies, are common in Mg₃Sb₂ and play a crucial role in shaping its thermoelectric properties, guiding experimental design and performance optimization. However, their impact on lattice thermal conductivity (κ_L) remains insufficiently understood. This work investigates the effects of Mg and Sb vacancies on the κ_L of Mg₃Sb₂ using a neural network potential (NNP). Our results show that both types of vacancies significantly reduce κ_L, primarily due to enhanced phonon-defect scattering. Comprehensive analyses of phonon dispersion, group velocities, mean square displacement (MSD), the phonon participation ratio (PPR), and elastic properties demonstrate that vacancies trigger pronounced phonon softening, slow down phonon transport, and promote strong localization, while simultaneously amplifying atomic vibrations and weakening interatomic bonding. This work clarifies the microscopic mechanisms by which point defects affect phonon transport and identifies defect engineering as an effective strategy for controlling thermal properties in thermoelectric materials.