Charge Disproportionation in SrBiFeO Containing Unusually High Valence Fe.

A Sr analogue of CaBiFeO, SrBiFeO, containing unusually high valence Fe ions was synthesized by using a high-pressure technique. It relieves the electronic instability due to the unusually high valence of Fe by a single charge disproportionation (CD) transition (Fe → 0.75Fe + 0.25Fe) rather than the successive CD and intermetallic charge transfer (CT) transitions seen in CaBiFeO. Conduction-band narrowing due to the significant bend in the Fe-O-Fe bond in the rhombohedral R3̅c crystal structure stabilized the charge-disproportionated state at low temperatures. Most importantly, Bi ions in SrBiFeO do not act as countercations accepting oxygen holes as they do in CaBiFeO, resulting in the absence of the intermetallic CT transition. The large cavity of the A-site Sr ions prevents the charge-transferred Bi from being stabilized. In the charge-disproportionated state the nearest-neighbor Fe spins align antiferromagnetically and one-fourth of the Fe spins are randomly replaced by Fe spins coupled ferromagnetically with the neighboring Fe spins.