Stoichiometry and properties of new calcium-ferrite-based phases

摘要

phase formation studies in the system Ca_2Fe2-xNb_xO_r: 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 Mossbauer 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 γto insulating at low γ.