Ultralow Thermal Conductivity in Vacancy‐Ordered Halide Perovskite Cs₃Bi₂Br₉ with Strong Anharmonicity and Wave‐Like Tunneling of Low‐Energy Phonons

Abstract

Halide perovskites are of great interest due to their exceptional optical and optoelectronic properties. However, thermal conductivity of many halide perovskites remains unexplored. In this study, an ultralow lattice thermal conductivity κL (0.24 W m−1 K−1 at 300 K) is reported and its weak temperature dependence (≈T−0.27) in an all‐inorganic vacancy‐ordered halide perovskite, Cs3Bi2Br9. The intrinsically ultralow κL can be attributed to the soft low‐lying phonon modes with strong anharmonicity, which have been revealed by combining experimental heat capacity and Raman spectroscopy measurements, and first‐principles calculations. It is shown that the highly anharmonic phonons originate from the Bi 6s2 lone pair expression with antibonding states of Bi 6s and Br 4p orbitals driven by the dynamic BiBr6 octahedral distortion. Theoretical calculations reveal that these low‐energy phonons are mostly contributed by large Br motions induced dynamic distortion of BiBr6 octahedra and large Cs rattling motions, verified by the synchrotron X‐ray pair distribution function analysis. In addition, the weak temperature dependence of κL can be traced to the wave‐like tunneling of phonons, induced by the low‐lying phonon modes. This work reveals the strong anharmonicity and wave‐like tunneling of low‐energy phonons for designing efficient vacancy‐ordered halide perovskites with intrinsically low κL.