On the relationships between structure, oxygen stoichiometry and ionic conductivity of CaTi1-xFexO3-δ (x=0.05, 0.20, 0.40, 0.60)

For the first time, Fe4+ was observed and analysed in a systematic manner in CaTi1-xFexO3-delta (x=0.05, 0.20, 0.40 and 0.60) perovskites and its presence related with the ionic conductivity of these materials. These oxides were prepared by solid state reaction under oxidizing conditions. Slowly cooled CaTi1-xFeO3-delta(x=0.05, 0.20 and 0.40) oxides crystallise with the same orthorhombic symmetry of CaTiO3. Some of the weaker peaks tend to disappear with increasing x and are no longer present for x=0.60, which can be indexed on a cubic unit cell. Mossbauer spectroscopy revealed the coexistence of Fe4+ and Fe3+ in all compositions, with prevalence of the Fe3+ species. The relative amounts of Fe3+ coordinated by 6, 5 and 4 oxygen ions were estimated. Oxygen stoichiometry changes, determined by solid electrolyte potentiometry-coulometry, were found to be in good agreement with the number of oxygen vacancies per unit formula based on estimates of the relative amounts of Fe4+ and Fe3+ obtained from the Mossbauer spectra. The ionic conductivity follows a typical Arrhenius behaviour with a sharp maximum at x=0.20. This behaviour is explained from combined Mossbauer spectroscopy and coulometric titration data, based on the existence of ordered and disordered oxygen vacancies associated with tetracoordinated and pentacoordinated Fe3+ ions, respectively. Differences between the ionic conductivity of ceramics prepared under reducing and oxidising conditions suggest that Fe4+ cations may stabilise disordered structures at, low temperatures, thus enhancing the transport properties. (C) 2003 Elsevier Science B.V. All rights reserved.