Coherent phonon tunneling-driven ultralow and non-monotonic thermal conductivity in quasi-0D Cs3Cu2I5

Low-dimensional copper halides have emerged as promising thermoelectric materials due to their phonon-glass electron-crystal behavior, yet their thermal transport mechanisms remain insufficiently understood. Using ab initio calculations and a unified thermal transport theory, we identify that Cs3Cu2I5 possesses an ultralow lattice thermal conductivity (κL) of 0.126 W/(m K) at room temperature (RT)—one of the lowest among metal halides. Above RT, coherent phonon tunneling dominates over particle-like propagation, resulting in glass-like κL in all directions. Intriguingly, the competing contributions of coherent and incoherent terms induce an anomalous non-monotonic temperature dependence of κL along the c axis, with an initial decrease followed by an unexpected rise. This behavior arises from strong anharmonicity and dense flat phonon dispersions, driven by hierarchical bonding and structural complexity. Our work uncovers unconventional heat transport mechanisms in complex crystals with strong anharmonicity.