Charge Disproportionation in Sr0.5Bi0.5FeO3 Containing Unusually High Valence Fe3.5+
P. Xiong, F. D. Romero, Y. Hosaka, H. Guo, T. Saito, W.-T. Chen, Y.-C. Chuang, H.-S. Sheu, G. McNally, J. P. Attfield*, and Y. Shimakawa*
2018/02/01
A Sr analogue of Ca0.5Bi0.5FeO3, Sr0.5Bi0.5FeO3, containing unusually high valence Fe3.5+ ions was synthesized by using a high-pressure technique. It relieves the electronic instability due to the unusually high valence of Fe3.5+ by a single charge disproportionation (CD) transition (Fe3.5+ → 0.75Fe3+ + 0.25Fe5+) rather than the successive CD and intermetallic charge transfer (CT) transitions seen in Ca0.5Bi0.5FeO3. 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, Bi3+ ions in Sr0.5Bi0.5FeO3 do not act as countercations accepting oxygen holes as they do in Ca0.5Bi0.5FeO3, resulting in the absence of the intermetallic CT transition. The large cavity of the A-site Sr ions prevents the charge-transferred Bi5+ from being stabilized. In the charge-disproportionated state the nearest-neighbor Fe3.5+ spins align antiferromagnetically and one-fourth of the Fe3.5+ spins are randomly replaced by Fe5+ spins coupled ferromagnetically with the neighboring Fe3.5+ spins.
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