Ultralow Lattice Thermal Conductivity in Double Perovskite Cs2AgRhCl6: The Effect of Anharmonic Phonon Renormalization and Wave-Like Phonon Tunneling

Abstract

Lead-free halide double perovskites are emerging as promising candidates for energy conversion applications, and elucidating their thermal transport mechanisms is essential for enhancing energy conversion efficiency. In this study, we employ first-principles calculations integrated with a unified phonon transport theory to systematically investigate the lattice thermal conductivity and phonon characteristics of the double perovskite Cs2AgRhCl6. Taking into account the contributions of particle-like and wave-like phonons, the room-temperature thermal conductivity of Cs2AgRhCl6 is reported to be 0.52 W/mK. It is demonstrated that cubic anharmonicity-induced phonon renormalization causes softening of phonon frequencies and shortens phonon lifetimes by increasing the scattering phase space of low-frequency phonons. The intrinsic strong anharmonicity coupled with phonon bundling effects facilitates significant wave-like phonon tunneling transport, which has a broader frequency distribution similar to the phonon density of states and dominates the total thermal conductivity at elevated temperatures. The suppression of high-frequency optical phonon by four-phonon scattering reduces particle-like thermal conductivity, while the concurrent increase in phonon linewidth enhances the wave-like component of thermal transport. These insights advance the understanding of anomalous thermal behavior in strongly anharmonic perovskites.