Ultralow Lattice Thermal Conductivity Induced by Unique Hierarchical Rotational Dynamics in Lightweight Cyanide‐Bridged Framework Materials

Abstract The pursuit of materials that combine light constituent elements with ultralow lattice thermal conductivity (κ L ) is crucial for advancing technologies like thermoelectrics and thermal barrier coatings; however, it remains a formidable challenge. Herein, ultralow κ L is achieved in lightweight cyanide‐bridged framework materials (CFMs) through the rational integration of properties such as the hierarchical vibrations exhibited in superatomic structures and the rotational dynamics exhibited in perovskites. Unique hierarchical rotational behavior results in multiple negative peaks in Grüneisen parameters across a wide frequency range, thereby inducing strong cubic anharmonicity and pronounced negative thermal expansion. The synergistic effect between the large four‐phonon scattering phase space and strong quartic anharmonicity leads to giant quartic anharmonic scattering rates. Consequently, the κ L of these CFMs decreases by one to two orders of magnitude with that of the known perovskites or perovskite‐like materials with equivalent average atomic masses. Cd(CN) 2 , NaB(CN) 4 , LiIn(CN) 4 , and AgX(CN) 4 (X = B, Al, Ga, In) exhibit ultralow room‐temperature κ L values ranging from 0.35 to 0.81 W m −1 K −1 . This study not only establishes CFMs as a novel platform for studying extreme phonon anharmonicity, but also provides a new paradigm for achieving ultralow κ L in lightweight materials via the conscious integration of hierarchical and rotational dynamics.