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 In₂G₂Se₆ is reported, in which the distorted InSe₆ octahedron forms monolayers in conjunction with Ge₂Se₆ 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, In₂Ge₂Se₆ exhibits ultralow out-of-plane thermal conductivity, κ_out-of-plane ≈0.2 W m^-1K^-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.