A family of oxygen-deficient perovskite phases with compositions, Ca<sub>2</sub>Mn<sub>2-x</sub>Nb<sub>x</sub>O<sub>γ</sub> 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 Ca<sub>2</sub>Mn<sub>2-x</sub>Nb<sub>x</sub>O<sub>γ</sub> 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 GdFeO<sub>3</sub>-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 GdFeO<sub>3</sub>-type distortion vary greatly with Mn valency, oxygen content and B-cation content. The orthorhombic GdFeO<sub>3</sub>-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 Mn<sup>3+</sup> does not result in cooperative distortions of (Mn,Nb)O<sub>6</sub> 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 t<sub>2g</sub> electrons are responsible for a variation of conductivity and activation energy rather than e<sub>g</sub> electrons. Conductivities are believed to depend mainly on the degree of π-orbital overlap between Mn and oxygen and hence, on interatomic distances. At low temperatures, the material exhibits spin glass-like behaviour.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:310732 |
| Date | January 1999 |
| Creators | Kruth, Angela |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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