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−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 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. 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 oxygen 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 is explained from combined Mössbauer 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.