Long‐Range Anion Correlations Mediating Dynamic Anharmonicity and Contributing to Glassy Thermal Conductivity in Well‐Ordered K₂Ag₄Se₃

Abstract Herein an extremely low (0.32‒0.25 Wm⁻¹K⁻¹) and glassy temperature‐dependence (300–600 K) of lattice thermal conductivity (κ_lat) in a monoclinic K₂Ag₄Se₃ is reported. It is found that the effective carrier delocalization, contributed by the perfect p−d^* hybridization paradigm, can efficiently facilitate the spatial transfer of electron cloud perturbations induced by the anisotropic thermal vibrations of Ag4 atoms, thereby favoring long‐range Se‒Se correlations. The localized rattling‐like vibration of Ag4 atoms induce short phonon lifetimes, large scattering phase space, and then a low particle‐like propagation. While the correlated interactions mediated competitive expressions between bubble diagrams and loop diagrams can suppress the generation of wavelike phonons from off‐diagonal coupling. Ultimately, the AgSe₃ structural units can enable the dual confinement of both the particle‐like propagation of phonons and wavelike tunneling of coherence. The study highlights that the correlated AgSe₃ coordination units can simultaneously target particle‐like and wavelike phonons and then reduce their contribution to the κ_lat by mediating long‐range transfer of charge polarization. These fundamental advances will advance the design of crystalline materials with tailored thermal properties.