Emerging Anion Disorder in CaTi_1-xFe_xO_3-x/2 Perovskites by X-ray Spectroscopy and Neutron Total Scattering

The long-range average and short-range local structures of the perovskite-type CaTi_1-xFe_xO_3-x/2 (0.00 ≤ x ≤ 0.40) oxides have been determined using high-resolution synchrotron X-ray diffraction (SXRD), neutron powder diffraction (NPD), and neutron pair distribution function (NPDF) methods, supported by X-ray absorption near-edge structure spectroscopy (XANES). Partial replacement of Ti⁴⁺ by Fe³⁺ leads to an increase in symmetry in the long-range average structure from orthorhombic Pbnm to cubic Pm3̅m. This also leads to the introduction of anion vacancies with disordering of the anion sublattice observed as broad and diffuse reflections in the NPD. Changes in the Fe L₃-edge features upon doping demonstrate a significant change in the Fe³⁺ coordination environment. However, only subtle changes are observed in the Ti L₃-edge spectra, suggesting that the oxygen vacancies are predominantly located around the Fe³⁺ cations. The NPDF analysis confirms that the anion vacancies sit preferentially around the Fe³⁺ cations, resulting in the formation of a range of differently sized FeO_n (n = 4, 5, 6) polyhedra and disordered polyhedral tilting. The change from a long-range orthorhombic Pbnm to a cubic Pm3̅m structure is strongly correlated with a change in the Ca²⁺ off-centering and displacement. We relate our findings to the previously reported ionic conductivities of these compositions for a complete structure-property relationship. This work has revealed local and engineerable structural complexities beyond the average structure and improves our understanding of the structure-property relationships of perovskites for application in solid-state ionic conductors and oxygen-transport membranes.