Structural evidence for the charge disproportionation of Fe4+ in BaFeO3-δ

A weak distortion of a [FeO6] octahedron was observed in hexagonal BaFeO2.91 at around 180 K by neutron powder diffraction. The occurrence of a weak distortion is consistent with the results of Mossbauer measurements, which indicated a decrease in the Fe4+ concentration below 170 K. We strongly suggest that the weak distortion of a [FeO6] octahedron results from a charge-disproportionation phenomenon, 2Fe(4+) --> Fe<4+tau>+ + Fe<4-tau>+, where 0 < tau less than or equal to 1. Below 130K, BaFeO2.91 orders antiferromagnetically and the propagation vector is <00 0.45c*> at 50K.