Anomalous variations of unit cell parameters with composition in proton conducting, ACeO3-type perovskite solid solutions

Abstract

Doped cerate perovskite oxides are potential candidates for application as proton conducting electrolytes in solid protonic fuel cells. Previous reports suggested nonsystematic variations of structural parameters in several doped cerate systems and that solid solution formation on substitution or removal of cations does not obey Vegard's law. In this paper, we show that the anomalies are, in fact, a wide-ranging phenomenon in this class of materials. Several different systems have been investigated with different compositions prepared under comparable conditions in air. In all cases, deviations of oxygen stoichiometry from that determined by cation stoichiometry were very small, <0.01% per perovskite formula unit. In ACe1−xMxO3−x/2 and A1−zCe0.9Y0.1O2.95−z, anomalies occur at oxygen stoichiometry 2.94, corresponding to an overall oxygen vacancy concentration of ca. 2%. In systems with constant oxygen vacancy concentrations, such as Ba1−xLaxCe0.9−xM0.1+xO2.95 and Ba1−xLaxCe0.85−xM0.15+xO2.925, structural parameters show anomalous variations at lanthanum contents around x=0.05. Possible factors that might lead to the occurrence of anomaly regions are discussed: oxygen vacancy ordering at vacancy concentrations above 1 in 48, cross-substitution of dopant ions at high doping levels or A-cation deficiency and the formation of defect clusters. Possibly, the most important factor may relate to the high degree of octahedral distortion associated with the unusual axial metric in most of the investigated compositions.