Avoided Crossing Phonons Realizes High‐Performance Single‐Crystalline β‐Zn₄Sb₃ Thermoelectrics

Abstract This study reveals the mechanisms behind the ultralow lattice thermal conductivity κ_L in β‐Zn₄Sb₃ single crystals through inelastic neutron scattering (INS). Analyzing phonon behaviors and the interaction between acoustic phonons and rattling modes, the first experimental evidence of avoided crossing in β‐Zn₄Sb₃ is provided. The rattler‐phonon avoided crossings contribute to the low κ_L in a β‐Zn₄Sb₃ single crystal, enhancing the thermoelectric figure‐of‐merit (zT). TEM characterizations of the β‐Zn₄Sb₃ single crystal with intrinsic and ultralow κ_L reveal a grain‐boundary‐free structure with uniformly dispersed rotation moiré fringes that contribute to low lattice thermal conductivity while maintaining a uniform elemental distribution. Additionally, the significant impact of crystallinity control coupled with dilute doping on boosting thermoelectric performance, with single‐crystalline single leg outperforming their polycrystalline counterparts is demonstrated. Notably, the conversion efficiency η of the undoped β‐Zn₄Sb₃ single leg achieves 1.4% under a temperature gradient of 200 K.