For the first time, Fe4+ was observed and analyzed in a systematic manner in CaTi1-xFexO3-δ (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-xFexO3-δ (x = 0.05, 0.20 and 0.40) oxides crystallize with the same orthorhombic symmetry of CaTiO3. Some of the weaker peaks tend to disappear with increasing x and were no longer present for x=0.60, which could be indexed on a cubic unit cell. Mössbauer 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 O vacancies per unit formula based on estimates of the relative amounts of Fe4+ and Fe3+ obtained from the Mössbauer spectra. The ionic conductivity follows a typical Arrhenius behaviour with a sharp maximum at x = 0.20. This behaviour was explained from combined Mössbauer spectroscopy and coulometric titration data, based on the existence of ordered and disordered O vacancies associated with tetra-coordinated and penta-coordinated Fe3+ ions, respectively. Differences between the ionic conductivity of ceramics prepared under reducing and oxidizing conditions suggested that Fe4+ cations may stabilize disordered structures at low temperatures, thus enhancing the transport properties.

On the Relationships between Structure, Oxygen Stoichiometry and Ionic Conductivity of CaTi1-xFexO3-δ (x = 0.05, 0.20, 0.40, 0.60). F.M.Figueiredo, J.Waerenborgh, V.V.Kharton, H.Näfe, J.R.Frade: Solid State Ionics, 2003, 156[3-4], 371-81