Covalency Competition in the Quadruple Perovskite CdCu₃Fe₄O₁₂

Abstract

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.