Covalency Competition in the Quadruple Perovskite CdCu₃Fe₄O₁₂

Cadmium ions (Cd²⁺) are similar to calcium ions (Ca²⁺) in size, whereas the Cd²⁺ ions tend to form covalent bonds with the neighboring anions because of the high electronegativity. The covalent Cd–O bonds affect other metal–oxygen bonds, inducing drastic changes in crystal structures and electronic states. Herein, we demonstrate high-pressure synthesis, crystal structure, and properties of a new quadruple perovskite CdCu₃Fe₄O₁₂. This compound exhibits an electronic phase transition accompanying a charge disproportionation of Fe ions without charge ordering below ∼200 K, unlike charge-disproportionation transition with rock-salt-type charge ordering for CaCu₃Fe₄O₁₂. First-principle calculations and Mössbauer spectroscopy display that covalent Cd–O bonds effectively suppress the Fe–O bond covalency, resulting in an electronic state different from that of CaCu₃Fe₄O₁₂. This finding proposes covalency competition among constituent metal ions dominating electronic states of complex metal oxides.