The role of the Eu3+/Eu2+ redox-pair in the electrical properties of Sr2EuNb1xTixO6 oxidesElectronic supplementary information (ESI) available: Fig. ESI 1 XRD patterns for Sr2EuNb1xTixO6x/2 with x = 0.25. Fig. ESI 2. Magnetic susceptibility as a function of temperature for as-prepared Sr2EuNb0.85Ti015O5.925 and samples reduced under different pO2 conditions. Table ESI 1: structural parameters for Sr2EuNb1xTixO6x/2 (0 x 0.20) compounds obtained from XRD. Table SI 2: selected structural informatio

In the search for new materials potentially useful as SOFC components perovskite-like oxides of the Sr 2 EuNb 1 x Ti x O 6 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 = 2 a p 8.28 Å, and S.G. P 2 1 / n . As required for SOFC materials, these oxides are stable under a wide range of oxygen partial pressures from the ambient condition to pO 2 10 30 atm. Aliovalent substitution of Nb 5+ by Ti 4+ 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, pO 2 dependence of conductivity and ion-blocking measurements demonstrate that the predominant conduction mechanism depends on the oxygen partial pressure. In the high pO 2 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 pO 2 increases. Under severe reducing conditions (pO 2 below 10 22 atm), n-type conduction dominates. Magnetic measurements demonstrate reduction of Eu 3+ occurs whereas the rest of the elements remain in their highest oxidation states. Thus, the Eu 3+ /Eu 2+ redox pair participates in the equilibrium defect responsible for n-type conduction. For intermediate pO 2 (10 20 to 10 8 atm) a significant pO 2 -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 Eu 3+ (this occurs at pO 2 as high as 10 10 atm). Oxides in the Sr 2 EuNb 1 x Ti x O 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. In the Sr 2 EuNb 1 x Ti x O 6 x /2 perovskite-like oxides the rare-earth ions play a role both in the structure and in the electrical behaviour.