Extended Antibonding States and Phonon Localization Induce Ultralow Thermal Conductivity in Low Dimensional Metal Halide

Abstract Thermal conductivity, which measures the ease at which heat passes through a crystalline solid, is controlled by the nature of the chemical bonding and periodicity in the solid. This necessitates an in‐depth understanding of the crystal structure and chemical bonding to tailor materials with notable lattice thermal conductivity (κ_L). Herein, the nature of chemical bonding and its influence on the thermal transport properties (2–523 K) of all‐inorganic halide perovskite Cs₃Bi₂I₉ are studied. The κ_L exhibits an ultralow value of ≈0.20 W m⁻¹K⁻¹ in 30–523 K temperature range. The antibonding states just below the Fermi level in the electronic structure arising from the interaction between bismuth 6s and iodine 5p orbitals, weakens the bond and causes soft elasticity in Cs₃Bi₂I₉. First‐principles density functional theory (DFT) calculations reveal highly localized soft optical phonon modes originating from Cs‐rattling and dynamic double octahedral distortion of 0D [Bi₂I₉]³⁻ in Cs₃Bi₂I₉. These low energy nearly flat optical phonons strongly interact with transverse acoustic modes creating an ultrashort phonon lifetime of ≈1 ps. While the presence of extended antibonding states gives rise to soft anharmonic lattice; Cs rattling provides sharp localized optical phonon modes, which altogether result in strong lattice anharmonicity and ultralow κ_L.