Bonding Heterogeneity and Nanoprecipitation on Substituting the Anionic Framework in Mg₃Sb₂ for p‐Type Zintl Thermoelectrics

Designing zintl compounds with complex crystal structures and large unit cells containing heavy elements may inherit bonding heterogeneity‐induced lattice anharmonicity, leading to intrinsically low thermal conductivity. In this work, alloying‐induced bonding heterogeneity in the extensively explored α‐Mg 3 (Sb, Bi) 2 phase due to local atomic ordering, site preferences, and prevailing heterogenous interfaces is evaluated in the p‐type Mg 3 (Sb 1−2 x Bi x Sn x ) 2 ‐based polyanionic nanocomposites for different alloying concentrations. The inherent susceptibility for partial phase transition (trigonal → monoclinic) is observed upon alloying, which is driven by alterations in bonding patterns, localized distortion, and secondary phase formation. At low alloying content ( x ≤ 0.05), a trigonal (Sb, Sn) phase is observed, while for higher alloying content ( x ≥ 0.1), a cubic Mg 2 Sn nanophase emerges. A synergistic reduction in thermal conductivity and enhanced power factor maximize the zT ≈ 0.25(±0.05) at 673 K in the optimized p‐type Mg 3 (Sb 0.9 Bi 0.025 Sn 0.025 ) 2 nanocomposites. The study highlights bonding heterogeneity‐induced structural transitions as an inherent challenge, where the dominant role of anionic sites becomes pivotal for determining/deriving favorable structural and functional properties in Mg 3 (Sb, Bi) 2 ‐based zintl compounds.