Intrinsic Relaxor Ferroelectric Driven Ultralow Glassy Thermal Conductivity in AgPbSbSe ₃

Crystalline solids exhibiting glass-like ultralow thermal conductivity while maintaining their long-range structural order are fundamental to structural chemistry, and they hold substantial potential in the field of thermoelectrics. Though often attributed to strong anharmonic interactions and complex crystal structures, the microscopic origin of the thermally insulating nature of these crystals remains unclear, necessitating an extensive evaluation of the chemical bonding, local structure, and phonon dynamics. Here, we uncover the emergence of glass-like thermal conductivity in the single crystal of AgPbSbSe 3, driven by its inherent relaxor ferroelectric behavior. Synchrotron X-ray pair distribution function (X-PDF) analysis reveals local structural symmetry breaking driven by Se off-centering along the crystallographic ⟨100⟩ direction, which constitutes the polar instability. Concurrently, density functional theory (DFT) calculations corroborate the experimental findings by identifying the antiparallel vibrations of cation (Ag) and anion (Se) sublattices and the associated double-well potential energy surface, generating dipole moments that underpin relaxor ferroelectric behavior in AgPbSbSe 3 . The presence of randomly oriented polar nanodomains, created by local atomic distortion in the structure, facilitates the structure to shuttle between randomly oriented, multiple nearly degenerate structures in the potential energy landscape, resulting in ultralow and glass-like thermal transport in AgPbSbSe 3 .