Octahedral Twisting‐Mediated van der Waals Stacking Induces Ultralow Thermal Conductivity

Low thermal conductivity ( κ ) is an important physical parameter inherent to all solids, and the quest for intrinsic ultralow‐ κ solids is one of the key scientific issues. Material design based on functional units can achieve control over lattice and phonon dynamics, and it is proposed that asymmetric structural units‐mediated van der Waals stacking can collectively lead to the low thermal conductivity. Herein, a novel van der Waals material In 2 G 2 Se₆ is reported, in which the distorted InSe₆ octahedron forms monolayers in conjunction with Ge 2 Se₆ dimers, which are further stacked along the c ‐axis via weak metavalent bonding. The distorted octahedron and van der Waals stacking result in large anharmonicity and ultrasoft acoustic phonon along Γ – Z direction, respectively. Consequently, In 2 Ge 2 Se 6 exhibits ultralow out‐of‐plane thermal conductivity, κ out‐of‐plane ≈0.2 W m −1 K −1 at 600 K. This study establishes a model for phonon physics that simultaneously enhances phonon‐phonon scattering and lowers the phonon group velocity, revealing the great potential of functional‐unit‐based material design for low‐ κ solids.