Four-Phonon Scattering and Wave-Like Phonon Tunneling Drive Glassy Ultralow Lattice Thermal Conductivity in Cs₂AgInCl₆

Lead-free halide double perovskites (HDPs) provide a promising platform for high-performance thermoelectric due to their intrinsically ultralow lattice thermal conductivity κ_l. In this study, we comprehensively investigate the lattice dynamics of Cs₂AgInCl₆ using first-principles calculations. By explicitly incorporating four-phonon scattering and wave-like phonon tunneling, we predict a κ_l of 0.52 W·m^-1·K^-1 with a remarkably weak temperature dependence (κ_l ∝ T^-0.31), confirming the intrinsically glass-like ultralow κ_l in Cs₂AgInCl₆. Further analyses reveal that hierarchical chemical bonds, loosely bonded rattling atoms and a mixed crystalline-liquid state collectively induce strong anharmonicity manifested in flat phonon modes. These factors dominate the glass-like thermal transport component of κ_l. This work uncovers the underlying mechanisms governing the unusual thermal transport properties in lead-free HDPs and offers guiding principles for designing novel energy conversion technologies.