Synthesis and characterisation of a novel oxygen-deficient manganese-based perovskite series

A family of oxygen-deficient perovskite phases with compositions, Ca2Mn2-xNbxOγ, has been synthesised and characterised using X-ray Powder Diffraction and Thermogravimetry. Property characterisation has included the study of electrical properties by Impedance Spectroscopy and the investigation of magnetic properties for one composition, x = 1.0. Some compositions were tested for possible application as electrode materials in gas sensors. The Ca2Mn2-xNbxOγ system, 0 ≤ x ≤ 1.2, with variable oxygen content, γ, can accommodate up to 20% vacancies at oxygen sites and Mn occurs in valence states ranging from +2 to +4. Depending on the B-cation ratio and oxygen content, two solid solution form: an extensive GdFeO3-type solid solution over the entire range of cation content, 0 ≤ x ≤ 1.2, with zero or small oxygen deficiencies and a closely-related, grossly oxygen-deficient solid solution over the range 0.3 ≤ x ≤ 0.8 with a simple cubic perovskite structure. Unit cell volume and orthorhombic GdFeO3-type distortion vary greatly with Mn valency, oxygen content and B-cation content. The orthorhombic GdFeO3-type structure of composition x = 1.0 was refined by the Rietveld method. Mn and Nb are disordered over the B-sites. Jahn-Teller activity of Mn3+ does not result in cooperative distortions of (Mn,Nb)O6 octahedra, but it was observed to effect the oxygen stoichiometry. The Nb-rich composition, x = 1.0, can accommodate large amounts of Ca-vacancies (up to 10 %). Electrical conductivity varies greatly with the composition. Results suggest that t2g electrons are responsible for a variation of conductivity and activation energy rather than eg electrons. Conductivities are believed to depend mainly on the degree of π-orbital overlap between Mn and oxygen and hence, on interatomic distances. Studies of magnetic properties were carried out for composition x = 1.0, which was paramagnetic at high temperatures. At low temperatures, the material exhibits spin glass-like behaviour. These new materials may be suitable as electrode materials on YSZ for electroanalysis of gases; some compositions showed a high sensitivity to propene, whereas others are good equilibrium electrodes. (author)