Emergent coherent-incoherent phonon crossover enables ultralow lattice thermal conductivity in 3D double perovskite.

The interplay between coherent (wave-like) and incoherent (particle-like) phonon transport is central to heat conduction, yet experimental evidence of their crossover in bulk three-dimensional (3D) crystals remains scarce. Here, we report intrinsically ultralow lattice thermal conductivity ([Formula: see text]) in the lead-free double perovskite Cs2AgBiCl6, reaching 0.52 and 0.28 W m-1 K-1 in single crystal and polycrystals, respectively, at room temperature. Combining thermal transport, low-temperature heat capacity, Raman spectroscopy, synchrotron x-ray pair distribution function analysis, and anharmonic lattice-dynamics calculations, we uncover the microscopic origin of this anomalous heat transport. Chemical bonding heterogeneity, weakly bound rattling-like Cs vibrations, anisotropic Cl dynamics, and soft lattice modes generate low-energy localized optical phonons that strongly interact with acoustic modes. Enhanced three- and four-phonon scattering suppresses phonon lifetimes, while anharmonic renormalization flattens optical branches and reduces group velocities. Coherent contributions account for ∼50% of [Formula: see text] near room temperature and exceeding incoherent contributions above ∼200 K, revealing an emergent coherent-incoherent phonon crossover in a fully crystalline 3D solid.