New oxygen-deficient perovskite series, Ca2Mn2-xNbxOy : 0 1.2

An extensive range of orthorhombic perovskites with the GdFeO3 structure, variable B-cation content and variable oxygen content form according to the formula Ca2Mn2-xNbxOgamma: 0 < x < 1.2. A second, closely-related, oxygen deficient solid solution with a simple cubic perovskite structure forms over the range 0.3 < x < 0.8. Oxygen contents range from 5.0 < gamma < 6.0 for x = 0 to 5.8 < gamma < 6.0 for x = 1.2 and are obtained by post-reaction heat treatment, either in air at different temperatures or in H-2-N-2. The orthorhombic solid solutions are either oxygen-stoichiometric or lightly reduced and transform to the cubic structure on more extensive reduction. A phase diagram is presented showing the equilibrium gamma values in air over the temperature range 1000-1350 degrees C. The variable composition of the solid solutions is accommodated by vacancies in up to 20% of the oxygen positions and variations in the oxidation state of Mn, between + 2 and + 4.