Glass-like Transport Dominates Ultralow Lattice Thermal Conductivity in Modular Crystalline Bi 4 O 4 SeCl 2 .

Crystalline Bi 4 O 4 SeCl 2 exhibits record-low 0.1 W/mK lattice thermal conductivity (κ L ), but the underlying transport mechanism is not yet understood. Using a theoretical framework which incorporates first-principles anharmonic lattice dynamics into a unified heat transport theory, we compute both the particle-like and glass-like components of κ L in crystalline and pellet Bi 4 O 4 SeCl 2 forms. The model includes intrinsic three- and four-phonon scattering processes and extrinsic defect and extended defect scattering contributing to the phonon lifetime, as well as temperature-dependent interatomic force constants linked to phonon frequency shifts and anharmonicity. Bi 4 O 4 SeCl 2 displays strongly anisotropic complex crystal behavior with dominant glass-like transport along the cross-plane direction. The uncovered origin of κ L underscores an intrinsic approach for designing extremely low κ L materials.