OXYGEN-DEFICIENT BROWNMILLERITES - CATION COORDINATION AND MAGNETIC-PROPERTIES

New oxygen-deficient brownmillerites Ca2Fe2-xMxO5- x 2 have been synthesized and characterized by X-ray diffraction and chemical analysis for M = Mg, Ni, Cu, Zn. The Mössbauer study of these phases shows only the presence of Fe(III) and confirms the oxygen non-stoichiometry; it suggests the existence of pyramidal sites and allows the ordered cationic distribution to be established according to the formula Ca2(Fe1111)Te(Fe111 1-2xM11x)oc (Fe111x)PyO5- x 2. The study of the magnetic susceptibility vs temperature shows a broad antiferromagnetic transition, with a classical decrease of TN vs x. A particular feature is the higher TN value of the magnesium phase compared with the nickel and copper oxides. This latter behavior is interpreted in terms of size factors influencing the interaction between the octahedral (or pyramidal) and tetrahedral layers. © 1990.