The role of the Eu³⁺/Eu²⁺ redox-pair in the electrical properties of Sr₂EuNb_1−xTi_xO_6−δ oxides

In the search for new materials potentially useful as SOFC components perovskite-like oxides of the Sr2EuNb1−xTixO6−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 = ap√2 ≈ 5.88 Å, c = 2ap ≈ 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 Sr2EuNb1−xTixO6−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.