Nanoscale Local Symmetry Optimization Induced Low Lattice Thermal Conductivity in I‐V‐VI ₂ Compounds

ABSTRACT The intrinsically low lattice thermal conductivity ( κ _L ) caused by lone pair electrons makes I‐V‐VI ₂ compounds promising candidates for thermoelectric applications. Elucidating the underlying mechanism of the correlation between lone pair electrons and defects plays an important role in providing critical insights for further designing low κ _L materials. This work employs X‐ray pair distribution function analysis to probe the nanoscale local atomic structures of I‐V‐VI ₂ compounds, using AgSbSe ₂ as the model material to understand the structure‐property relationship. The stereochemically active lone pair electrons on Sb ³⁺ induce significant off‐centering displacements, introducing local symmetry breaking that can be systematically tuned through controlled vacancies, atomic disordering, or substitution. Theoretical calculations validate the connection between structural distortions and chemical bonding strength, resulting in the change of κ _L . This work not only unveils the local structure evolution of AgSbSe ₂ during alloying, but also provides a roadmap for performance optimization of I‐V‐VI ₂ compounds with lone pair electron effects.