Glassy thermal conductivity in Cs3Bi2I6Cl3 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, K-L, decreases, and its fundamental characteristics changes. The realization of ultralow and unusual glass-like K-L in a crystalline material is challenging but crucial to many applications like thermoelectrics and thermal barrier coatings. Herein, we demonstrate an ultralow (similar to 0.20 W/m.K at room temperature) and glass-like temperature dependence (2-400 K) of K L in a single crystal of layered halide perovskite, Cs3Bi2I6Cl3. 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 K-L is intriguing from lattice dynamical perspective as well as have potential applications.