Abstract
Phase formation studies in the system Ca 2Fe 2− x Nb x O γ :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 Fe 4+ and Fe 3+ 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 Ca 2FeNbO 6 has the GdFeO 3 structure. Atomic positions and occupancies were determinated by Rietveld refinements. Mössbauer analysis revealed only the presence of Fe 3+ in this sample. Although Ca 2FeNbO 6 was heated under a range of conditions, its oxygen content could not be modified. The conductivity of Ca 2FeNbO 6 corresponds to that of an insulator material. Magnetic susceptibility data indicated an antiferromagnetic behavior.
Published Version
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