Structural Evidence for the Charge Disproportionation of Fe 4+ in BaFeO 3-δ

A weak distortion of a [FeO 6 ] octahedron was observed in hexagonal BaFeO 2.91 at around 180 K by neutron powder diffraction. The occurrence of a weak distortion is consistent with the results of Mössbauer measurements, which indicated a decrease in the Fe 4+ concentration below 170 K. We strongly suggest that the weak distortion of a [FeO 6 ] octahedron results from a charge-disproportionation phenomenon, 2Fe 4+ → Fe <4+τ>+ + Fe <4-τ>+ , where 0<τ≤1. Below 130 K, BaFeO 2.91 orders antiferromagnetically and the propagation vector is <00 0.45 c * > at 50 K.