Cation Mg-Dominated Coherent Phonon Transport Leads to Anomalous Thermal Conductivity in Mg3Bi2

N -type Mg 3 (Sb,Bi) 2 system has emerged as a groundbreaking thermoelectric (TE) material, renowned for its optimized electronic transport properties and intrinsically low lattice thermal conductivity (κ l ). However, the origin of anomalously low κ l and its weak temperature scaling (κ l ∼ T −0.5 , deviating from the classical T −1 behavior) remain elusive. Employing a theoretical framework that integrates ab initio anharmonic lattice dynamics into a unified heat transport theory accounting for particle-like propagation (κ p ) and wave-like coherent (κ c ) transport, phonon renormalization, and fourth-order anharmonic phonon scattering, the thermal transport property of Mg 3 Bi 2 is comprehensively investigated. It is found that the weakly bonded interlayer cation Mg atoms amplify phonon coherence through dynamic disorder, driving wave-like transport contributes ≈40% to κ l at 600 K, while four-phonon scattering significantly suppresses particle-like propagation (>40% reduction). The theoretical predictions of both the magnitude and temperature-dependent behavior of κ l in Mg 3 Bi 2 exhibit synergistic agreement with in-house and reported experimental values. This work pioneers key insights into the Mg cation sublattice tuning the particle-wave duality in phonon transport, its strong anharmonic potential disrupts phonon phase coherence while enhancing four-phonon scattering, collectively driving the low κ l in Mg 3 Bi 2 . This discovery provides two actionable design strategies—cation site engineering and coherent phonon tuning—for developing high- zT Mg 3 Bi 2 -based thermoelectrics.