Lattice distortion leads to glassy thermal transport in crystalline Cs₃Bi₂I₆Cl₃

The glassy thermal conductivities observed in crystalline inorganic perovskites such as Cs₃Bi₂I₆Cl₃ is perplexing and lacking theoretical explanations. Here, we first experimentally measure such its thermal transport behavior from 20~K to 300~K, after synthesizing Cs₃Bi₂I₆Cl₃ single crystals. Using path-integral molecular dynamics simulations driven by machine learning potentials, we reveal that Cs₃Bi₂I₆Cl₃ has large lattice distortions at low temperatures, which may be related to the large atomic size mismatch. Employing the regularized Wigner thermal transport theory, we reproduce the experimental thermal conductivities based on lattice-distorted structures. This study thus provides a framework for predicting and understanding glassy thermal transport in materials with strong lattice disorder.