Effect of divalent cation addition on structure, conductivity and grain boundary properties in La doped ceria oxygen ion conductors

The aim of this work is to investigate the effect of divalent cations on the structure and electrical properties of Ce0.85La0.1D0.05O2-delta (D = Ca, Sr and Ba) oxygen ion conductors. The X-Ray structural analysis confirms the presence of CeDO3 minor phase in addition to cubic fluorite phase of ceria in Sr2+ and Ba2+ added compositions. The lattice parameter of the compositions significantly depends on the ionic radius of dopants and the presence of D2+ ions in ceria lattice. The Ca2+ added composition shows the highest free oxygen vacancy concentration due to its lowest association energy and complete dissolution of Ca2+ ions into ceria lattice. The dopant-vacancy association energy and grain interior conductivity changes with the ionic radii of the divalent dopants. The grain boundary capacitance depends on dielectric constant, grain size and grain boundary thickness. The grain boundary conductivity shows 46% over total conductivity for Sr2+ added composition. The presence of CeDO3 phase and space charge layer promotes the grain boundary resistances and affects the ion dynamics. Schematic models are proposed to understand the ion migration in grain boundaries. The scavenging effect is found to be highest in Sr2+ ions added composition. The defect structures, the presence of CeDO3 phase and electrical properties are correlated with each other.