Glassy thermal conductivity in CsBiICl 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, decreases, and its fundamental characteristics changes. The realization of ultralow and unusual glass-like κ 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 in a single crystal of layered halide perovskite, CsBiICl. Acoustic phonons with low cut-off frequency (20 cm) are responsible for the low sound velocity in CsBiICl 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 is intriguing from lattice dynamical perspective as well as have potential applications.