Preferential substitution at cationic sites in 9-4-9 Zintl phase Ca_9−yEu_yZn_4.5+δSb₉

Ca₉Zn_4.5Sb₉ Zintl compound is one of the promising thermoelectric materials with excellent performance, which can be further improved by Eu alloying. In this work, a series of Ca_9−yEu_yZn_4.5+δSb₉ compounds is investigated by using the advanced C_s-corrected high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) imaging technique at an atomic level. The HAADF-STEM images show that alloying Eu will lead to preferential substitution of Eu atoms at Ca(1) and Ca(5) sites. The further structural analysis and first-principles density functional theory calculation elucidate the origin that the Eu atoms are prone to replace Ca(1) and Ca(5) sites in Ca_9−yEu_yZn_4.5+δSb₉. The preferential substitutions may provide a new insight into revealing the structural transformation commonly found in Zintl phase thermoelectric materials.