Intrinsic Zn Vacancies-Induced Wavelike Tunneling of Phonons and Ultralow Lattice Thermal Conductivity in Zintl Phase Sr₂ZnSb₂

Understanding and quantifying the ultralow lattice thermal conductivity (κL) with anomalously weak temperature dependence in thermoelectrics play a critical role in optimizing the performance of energy-conversion devices. However, the role of high-order anharmonicity in suppressing κL and the validity of conventional phonon gas model in describing glasslike thermal transport remain unexplored in many systems with complex structures. Herein, with first-principles-based machine-learning potentials, we systematically investigate the anharmonic lattice dynamics and thermal transport properties in Zintl compounds SrCuSb and Sr2ZnSb2. Surprisingly, we demonstrate an insignificant reduction of κL from the quartic anharmonicity in Sr2ZnSb2, despite the strong four-phonon scattering in Zn-dominated frequency ranges. We show that the anharmonic phonon broadening in Sr2ZnSb2 is strong enough to break the particle-like description of κL and promote a nontrivial mutual coherence for phonon branches with large differences in frequencies, whereas in SrCuSb, such coupling is negligible. Our results further resolve the discrepancy between the theoretically predicted and experimentally measured κL in Sr2ZnSb2 at low temperatures by manipulating the intrinsic Zn vacancy ordering. The new physical insights obtained in this work pave the way to better understand glasslike κL in partially filled Zintl compounds with strong anharmonicity.