On the relationships between structure, oxygen stoichiometry and ionic conductivity of CaTi1−xFexO3−δ (x=0.05, 0.20, 0.40, 0.60)
Abstract For the first time, Fe 4+ was observed and analysed in a systematic manner in CaTi 1− x Fe x O 3− 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 CaTi 1− x Fe x O 3− δ ( x =0.05, 0.20 and 0.40) oxides crystallise with the same orthorhombic symmetry of CaTiO 3 . 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 Fe 4+ and Fe 3+ in all compositions, with prevalence of the Fe 3+ species. The relative amounts of Fe 3+ 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 Fe 4+ and Fe 3+ 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 Fe 3+ ions, respectively. Differences between the ionic conductivity of ceramics prepared under reducing and oxidising conditions suggest that Fe 4+ cations may stabilise disordered structures at low temperatures, thus enhancing the transport properties.