Control of bond-strain-induced electronic phase transitions in iron perovskites

Unusual electronic phase transitions in the A-site ordered perovskites LnCu₃Fe₄O₁₂ (Ln: trivalent lanthanide ion) are investigated. All LnCu₃Fe₄O₁₂ compounds are in identical valence states of Ln³⁺Cu²⁺₃Fe ^3.75+₄O₁₂ at high temperature. LnCu ₃Fe₄O₁₂ with larger Ln ions (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb) show an intersite charge transfer transition (3Cu²⁺ + 4Fe^3.75+ → 3Cu³⁺ + 4Fe³⁺) in which the transition temperature decreases from 360 to 240 K with decreasing Ln ion size. In contrast, LnCu₃Fe₄O₁₂ with smaller Ln ions (Ln = Dy, Ho, Er, Tm Yb, Lu) transform into a charge-disproportionated (8Fe ^3.75+ → 5Fe³⁺ + 3Fe⁵⁺) and charge-ordered phase below ∼250-260 K. The former series exhibits metal-to-insulator, antiferromagnetic, and isostructural volume expansion transitions simultaneously with intersite charge transfer. The latter shows metal-to-semiconductor, ferrimagnetic, and structural phase transitions simultaneously with charge disproportionation. Bond valence calculation reveals that the metal-oxygen bond strains in these compounds are classified into two types: overbonding or compression stress (underbonding or tensile stress) in the Ln-O (Fe-O) bond is dominant in the former series, while the opposite stresses or bond strains are found in the latter. Intersite charge transfer transition temperatures are strongly dependent upon the global instability indices that represent the structural instability calculated from the bond valence sum, whereas the charge disproportionation occurs at almost identical temperatures, regardless of the magnitude of structural instability. These findings provide a new aspect of the structure-property relationship in transition metal oxides and enable precise control of electronic states by bond strains.