Engineering Hidden Ferroelectric-Like Distortion for Ultralow Thermal Conductivity in Polar van der Waals Tetradymite

Ferroelectric-like polar structures enable diverse functionalities through coupling with charge and phonon transport. Although effective in modulating thermal conductivity, the rational design of novel polar materials with intrinsically low thermal conductivity via ferroelectric-like distortion engineering remains challenging. Here, we report the design of polar van der Waals (vdW) tetradymite systems, employing the s-p hybridization as a descriptor for antibonding-driven polar distortions. By engineering a polar layer of SnTe into a globally centrosymmetric R (Formula presented.) m matrix, we realize a polar-tetradymite SnBi₂Te₄ featuring hidden local distortions arising from Sn²⁺ off-centering, driven by stereochemically active 5s² lone pairs. Neutron total scattering reveals an average local polar displacement of approximately 0.11 Å, whose inversion-symmetry breaking features are directly confirmed by the second-harmonic generation measurement. These local ferroelectric-like distortions may contribute to low-lying optical phonon modes, as evidenced by low-temperature heat-capacity measurements, resulting in an ultralow lattice thermal conductivity of ∼0.61 W m⁻¹K⁻¹ at room temperature. This work establishes local symmetry breaking as an effective strategy for designing polar systems with intrinsically low thermal conductivity, offering promising opportunities for applications in thermoelectrics, field-effect transistors, and beyond.