Wavelike tunneling of phonons dominates glassy thermal conductivity in crystalline Cs₃Bi₂I₆Cl₃

Intrinsically low lattice thermal conductivity kappa(L) in halide perovskites is of great interest for energy conversion applications. Here, based on first-principles calculations, we systematically study the lattice thermal conductivity of the recently synthesized layered perovskite Cs3Bi2I6Cl3 . By using renormalized force constants extracted from lattice dynamics, our calculated kappa(L) is 0.227 and 0.130 Wm -1K-1 along the in-plane and cross-plane directions at 300 K, respectively, which agrees well with the experimental values (0.223 and 0.209 Wm -1K-1 parallel and perpendicular to the Bridgman growth direction). Meanwhile, kappa(L) follows a nonstandard kappa(L)proportional to T-0.237 dependence on heating, originating from the dual particle-wave behavior of heat-carrying phonons where wavelike tunneling dominates >72 % of the contribution to the total kappa(L) when T > 300 K. Further analyses imply that Cs3Bi2I6Cl3 manifests the coexistence of metavalent bonding, loosely bonded rattling atoms with thermally induced large-amplitude vibrations, and stereochemical lone pair activity, which induces strong anharmonicity with the soft low-lying modes, causes a mixed crystalline-liquid state, and, finally, produces unexpectedly glassy thermal conductivity. Our work pinpoints the microscopic origin of ultralow kappa(L) in Cs3Bi2I6Cl3 , which is important for designing efficient materials in halide perovskites for energy conversion.