A-site-cation deficiency in the SrCe0.9Yb0.1O3−δ perovskite: effects of charge-compensation mechanism on structure and proton conductivity

Abstract

Cation non-stoichiometry is now recognised to have a significant influence on the electrical-transport properties of proton-conducting oxides. The effects of A-site-cation deficiency in the Sr_1−xCe_0.9Yb_0.1O_3−δ perovskite system with potential use in electrochemical devices are examined. Electron diffraction is used to verify the orthorhombic perovskite structure (space group, Pnma) of compositions 0 ≤ x ≤ 0.06. Combined Rietveld refinements of high-resolution neutron and X-ray diffraction data confirm a previously reported anomalous evolution of the lattice parameters within the series, with a maximum in specific free volume observed for x = 0.04. Analysis of the ion-site occupancies indicates that A-site and oxygen vacancies form up to x = 0.04, with partial partitioning of the Yb dopant over the Sr and Ce sites taking place for higher levels of Sr deficiency. The change in defect chemistry is likely to be responsible for the anomalous evolution in structural parameters, and may arise from the alleviation of strain associated with substantial octahedral tilting which occurs for a high A-site vacancy content. In addition to lattice parameters, electrical transport and stability in CO₂ are also affected by dopant partitioning. The proton conductivity appears to be well correlated with both the nominal oxygen-vacancy content and the specific free volume. The stability of the series towards carbonation is not correlated to the Sr chemical potential but, instead, is lowest for the composition of greatest specific free volume.