Low Energy Shear Vibrations of Sb₂ Bilayer Driving Ultralow Lattice Thermal Conductivity in Homologous (Sb₂)_m(Sb₂Te₃)_n

Abstract Achieving ultralow lattice thermal conductivity (κ L ) in topological quantum materials with understanding of its origin poses a formidable challenge in material design. Members of the (Sb 2 ) m (Sb 2 Te 3 ) n (m, n: integers) homologous series, Sb 2 Te 3 , SbTe, Sb 2 Te, and Sb 4 Te 3 , exhibit natural van der Waals‐like heterostructure and maintain topologically protected surface states. This offers a unique platform for probing the modulation of κ L in conjunction with their local structure and lattice dynamics. We focus on three distinct members, SbTe, Sb 2 Te, and Sb 4 Te 3 , distinguished by different stacking sequences of Sb 2 bilayers (BLs) and Sb 2 Te 3 quintuple layers. Synchrotron X‐ray pair distribution function analysis reveals notable local structural signatures, distinguishing each compound. We observe a systematic κ L reduction across the series along layered stacking direction, with Sb 4 Te 3 exhibiting the lowest κ L (≈0.29 W m −1 K −1 at 300 K) due to enhanced phonon scattering from superlattice‐like heterostructure induced by BLs, while Sb 2 Te 3 having no BL retains the highest κ L (≈0.87 W m −1 K −1 at 300 K). Phonon modes dominated by low‐energy shearing vibrations of Sb 2 BLs couple with acoustic phonons, reducing phonon group velocity and suppressing heat transport. This study underscores the interplay of structural modularity and low‐energy selective lattice vibrations in achieving ultralow κ L in topological quantum materials.