Intrinsically low lattice thermal conductivity ${\kappa }_{\mathrm{L}}$ 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 ${\mathrm{Cs}}_3{\mathrm{Bi}}_2{\mathrm{I}}_6{\mathrm{Cl}}_3$. By using renormalized force constants extracted from lattice dynamics, our calculated ${\kappa }_{\mathrm{L}}$ is 0.227 and 0.130 ${\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ along the in-plane and cross-plane directions at 300 K, respectively, which agrees well with the experimental values (0.223 and 0.209 ${\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ parallel and perpendicular to the Bridgman growth direction). Meanwhile, ${\kappa }_{\mathrm{L}}$ follows a nonstandard ${\kappa }_{\mathrm{L}}\propto T^{-0.237}$ 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 ${\kappa }_{\mathrm{L}}$ when $T>$ 300 K. Further analyses imply that ${\mathrm{Cs}}_3{\mathrm{Bi}}_2{\mathrm{I}}_6{\mathrm{Cl}}_3$ 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 ${\kappa }_{\mathrm{L}}$ in ${\mathrm{Cs}}_3{\mathrm{Bi}}_2{\mathrm{I}}_6{\mathrm{Cl}}_3$, which is important for designing efficient materials in halide perovskites for energy conversion.

• Received 1 June 2023
• Accepted 17 November 2023

DOI:https://doi.org/10.1103/PhysRevB.108.224302