Stoichiometry and properties of new calcium–ferrite-based phases

Phase formation studies in the system Ca2Fe2−xNbxOγ:0⩽x⩽2.0 indicated the presence of two new phases, a cubic solid solution with a variable compositional extent x from 0.45⩽x⩽0.65 and an orthorhombic phase for x=1.0. Cubic solid solution has variable oxygen content with an oxygen-deficient perovskite structure. The oxidation state of Fe was investigated by Mössbauer spectroscopy; as synthesized, for x=0.6 it contains a mixture of Fe4+ and Fe3+ in the ratio 13/87. Its oxygen content can be modified over the range 5.55<γ<5.89 by heat treatments under a range of conditions. For most γ values, the crystal structure was that of a cubic perovskite; at high oxygen contents, γ>5.7, transformation to an orthorhombic structure was observed. Electrical properties range from semiconducting at high γ to insulating at low γ. The orthorhombic phase Ca2FeNbO6 has the GdFeO3 structure. Atomic positions and occupancies were determinated by Rietveld refinements. Mössbauer analysis revealed only the presence of Fe3+ in this sample. Although Ca2FeNbO6 was heated under a range of conditions, its oxygen content could not be modified. The conductivity of Ca2FeNbO6 corresponds to that of an insulator material. Magnetic susceptibility data indicated an antiferromagnetic behavior.