Successive Charge Transitions of Unusually High‐Valence Fe^3.5+: Charge Disproportionation and Intermetallic Charge Transfer

Abstract

A perovskite-structure oxide containing unusually high-valence Fe^3.5+ was obtained by high-pressure synthesis. Instability of the Fe^3.5+ in Ca_0.5 Bi_0.5 FeO₃ is relieved first by charge disproportionation at 250 K and then by intermetallic charge transfer between A-site Bi and B-site Fe at 200 K. These previously unobserved successive charge transitions are due to competing intermetallic and disproportionation charge instabilities. Both transitions change magnetic and structural properties significantly, indicating strong coupling of charge, spin, and lattice in the present system.