Phonon Coherence in Bismuth‐Halide Perovskite Cs₃Bi₂Br₉ With Ultralow Thermal Conductivity

Abstract Halide perovskites emerge as promising candidates for thermoelectrics due to their ultralow thermal conductivity. The conventional theory based on the phonon gas model, which treats thermal transport as particle‐like behavior, shows limitations to describe the unusual thermal transport property in some halide perovskites with strong anharmonicity. Here, the significance of phonon coherence effect on thermal transport of bismuth‐halide perovskite Cs 3 Bi 2 Br 9 is reported by inelastic neutron scattering and simulations including density functional theory and machine‐learning potential based molecular dynamics. This study shows that the restrictive low‐energy acoustic phonons lead to the limited particle‐like thermal conductivity, which seriously underestimates the lattice thermal conductivity of Cs 3 Bi 2 Br 9 . The significant contribution of wave‐like optical phonon modes, driven by the coherence effect, accounts for an additional ≈50% wave‐like thermal conductivity. Besides, the experimental weak temperature dependence of thermal conductivity along z direction ( κ ≈ T −0.35 ) is well reproduced by calculation ( κ ≈ T −0.37 ) when including phonon coherence. This work highlights the critical role of phonon coherence in Cs 3 Bi 2 Br 9 and enhances understanding on the unusual thermal transport properties in halide perovskites and other related materials with strong anharmonicity.