Avoided Crossing Phonons Realizes High‐Performance Single‐Crystalline <i>β</i>‐Zn₄Sb₃ Thermoelectrics

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.