The role of the Eu3+/Eu2+ redox-pair in the electrical properties of Sr2EuNb1−xTixO6−δ oxides

Abstract

In the search for new materials potentially useful as SOFC components perovskite-like oxides of the Sr₂EuNb_1−xTi_xO_6−δ series have been obtained, the solubility limit being ca. x = 0.15. Rietveld refinements of XRD data and SAED and HRTEM have demonstrated that these compounds are monoclinic with cell parameters a ≈ b = a_p√2 ≈ 5.88 Å, c = 2a_p ≈ 8.28 Å, and S.G. P2₁/n. As required for SOFC materials, these oxides are stable under a wide range of oxygen partial pressures from the ambient condition to pO₂ ≈ 10⁻³⁰ atm. Aliovalent substitution of Nb⁵⁺ by Ti⁴⁺ improves the electrical conductivity in air by two orders of magnitude for the end-member of the series (x = 0.15) compared with the parent material. Magnetic measurements, pO₂ dependence of conductivity and ion-blocking measurements demonstrate that the predominant conduction mechanism depends on the oxygen partial pressure. In the high pO₂ region (from 10⁻⁵ to 0.21 atm) p-type conduction is dominant due to the presence of oxygen vacancies which are being annihilated as pO₂ increases. Under severe reducing conditions (pO₂ below 10⁻²² atm), n-type conduction dominates. Magnetic measurements demonstrate reduction of Eu³⁺ occurs whereas the rest of the elements remain in their highest oxidation states. Thus, the Eu³⁺/Eu²⁺ redox pair participates in the equilibrium defect responsible for n-type conduction. For intermediate pO₂ (10⁻²⁰ to 10⁻⁸ atm) a significant pO₂-independent ionic conduction, due to the motion of anionic vacancies, is the dominant conduction mechanism, though a minor electronic n-type contribution is also observed, associated with the reduction of Eu³⁺ (this occurs at pO₂ as high as 10⁻¹⁰ atm). Oxides in the Sr₂EuNb_1−xTi_xO_6−x/2 series constitute an interesting example of rare-earth perovskites in which the rare-earth ions play a role not only in the structural but also in the electrical behaviour.