Observation of Temperature Dependent Atomic Off‐Centering in High Entropy Cubic I‐V‐VI₂/IV‐VI Alloys with Ultralow Thermal Conductivity

Abstract I‐V‐VI 2 /IV‐VI compounds and their alloys are of great interest in the field of thermoelectricity due to their unique high‐symmetry crystal structures and unexpectedly low thermal conductivity, which is closely related to the off‐centering effect in the cubic lattice. However, the influence of such an effect on thermal transport across different temperatures remains largely elusive. Herein, using in situ pair distribution function analysis, it is directly observed that the sustained low thermal conductivity in n‐type (AgBiSe 2 ) 1‐ x (PbS) x alloys stem from the dynamic intelligent switching of local structural distortions over wide temperature windows for the first time. Under ambient conditions, the local off‐centering of central atoms dominates the phonon scattering, while at high temperatures, the geometrical configuration of the ligands predominantly modulates thermal conductivity due to lattice expansion and diminished cation off‐centering. The complementary synergistic effect leads to an ultralow thermal conductivity (<0.64 W m −1 K −1 ) for (AgBiSe 2 ) 0.65 (PbS) 0.35 across the entire working temperature range and yields a lattice thermal conductivity of only 0.33 W m −1 K −1 at 800 K approaching the glass limit. Atomic‐scale insights into the thermal conductivity mechanism of I‐V‐VI 2 /IV‐VI alloys provide important guidance for designing high‐efficiency thermoelectric materials over broad operational temperature ranges.