New oxygen-deficient perovskite series, Ca2Mn2–xNbxOγ: 0<x<1.2

Abstract

An extensive range of orthorhombic perovskites with the GdFeO ₃ structure, variable B-cation content and variable oxygen content form according to the formula Ca ₂ Mn _2–x Nb _x O _γ : 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<γ<6.0 for x=0 to 5.8 <γ<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 γ values in air over the temperature range 1000-1350 °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.