Ultralow intrinsic lattice thermal conductivity induced by rattling behavior and dynamic rotation in the hexagonal CsMgX3 (X = Cl, Br, I)

Lead-free halide perovskites have attracted significant attention as promising thermoelectric materials due to their ultralow lattice thermal conductivity (κL). In this study, the phonon transport properties of hexagonal CsMgX3 (X = Cl, Br, I) are investigated using first-principles calculations combined with the unified theory of phonon thermal transport. The results indicate that the extremely weak Cs–X bond induces pronounced “rattling” behavior in the vibrations of Cs atoms, thereby driving strong phonon anharmonic scattering and sharply suppressing the thermal transport properties of CsMgX3. Therefore, the three-phonon κL (κ3php) values of CsMgCl3, CsMgBr3, and CsMgI3 are only 1.52, 0.76, and 0.40 W/mK, respectively. These values are further suppressed by four-phonon scattering, with κ3,4php decreasing to 1.22, 0.61, and 0.31 W/mK, respectively, at 300 K. This remarkable four-phonon effect primarily originates from the dynamic rotation of the [MgX6]4− octahedra. More importantly, the wave-like phonon tunneling plays no dominant role in the ultralow κL of these materials. The total κL (including both particle-like and coherent contributions, denoted as κ3,4php+c) increases merely slightly to 1.33, 0.68, and 0.37 W/mK. Our work thoroughly elucidates the lattice dynamics of hexagonal CsMgX3 and promises to shed light on the exploration and design of materials with ultralow intrinsic κL.