Glassy thermal conductivity in Cs 3 Bi 2 I 6 Cl 3 single crystal.

As the periodic atomic arrangement of a crystal is made to a disorder or glassy-amorphous system by destroying the long-range order, lattice thermal conductivity, κ L , decreases, and its fundamental characteristics changes. The realization of ultralow and unusual glass-like κ L in a crystalline material is challenging but crucial to many applications like thermoelectrics and thermal barrier coatings. Herein, we demonstrate an ultralow (~0.20 W/m·K at room temperature) and glass-like temperature dependence (2-400 K) of κ L in a single crystal of layered halide perovskite, Cs 3 Bi 2 I 6 Cl 3 . Acoustic phonons with low cut-off frequency (20 cm -1 ) are responsible for the low sound velocity in Cs 3 Bi 2 I 6 Cl 3 and make the structure elastically soft. While a strong anharmonicity originates from the low energy and localized rattling-like vibration of Cs atoms, synchrotron X-ray pair-distribution function evidence a local structural distortion in the Bi-halide octahedra and Cl vacancy. The hierarchical chemical bonding and soft vibrations from selective sublattice leading to low κ L is intriguing from lattice dynamical perspective as well as have potential applications.