Long-Range Anion Correlations Mediating Dynamic Anharmonicity and Contributing to Glassy Thermal Conductivity in Well-Ordered KAgSe.

Herein an extremely low (0.32‒0.25 WmK) and glassy temperature-dependence (300-600 K) of lattice thermal conductivity (κ) in a monoclinic KAgSe 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 AgSe 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 AgSe coordination units can simultaneously target particle-like and wavelike phonons and then reduce their contribution to the κ by mediating long-range transfer of charge polarization. These fundamental advances will advance the design of crystalline materials with tailored thermal properties.