Halide Perovskite: A Rich Source of Thermal Insulator.

Low lattice thermal conductivity is a key physical parameter for realizing efficient thermal management and energy conversion. Halide perovskites have emerged as an ideal platform for exploring the physics of extreme thermal transport and for designing novel thermal management materials, owing to their rich structural tunability and intrinsically ultralow thermal conductivity. This review discusses the origins of ultralow thermal conductivity in halide perovskites, spanning from macroscopic thermal phenomena to microscopic phonon transport. Halide perovskites exhibit characteristic thermal signatures, including weak temperature dependence of thermal conductivity, a boson-like peak in heat capacity, and low sound velocities. These properties stem from a soft crystal lattice associated with metavalent bonding, as well as from strong anharmonic phonon scattering induced by lattice disorder and rattling modes. Such glass-like lattice dynamics lead to an anomalous accumulation of low-frequency phonons and intense phonon scattering, pushing phonons toward the Ioffe-Regel limit and causing a breakdown of the conventional phonon gas model. Therefore, the inclusion of coherent phonons is essential for properly describing the intrinsic "phonon glass" character of these materials.