It was noted that previous reports had suggested a non-systematic variations of structural parameters in several doped cerate systems, and that solid solution formation upon substitution or removal of cations did not obey Vegard's law. It was shown here that the anomalies were a widespread phenomenon in this class of materials. Several systems were investigated, with different compositions prepared under comparable conditions in air. In all cases, deviations of O stoichiometry from that determined by cation stoichiometry were very small: less than 0.01% per perovskite formula unit. In ACe1−xMxO3−x/2 and A1−zCe0.9Y0.1O2.95−z, anomalies occurred at O stoichiometry 2.94, corresponding to an overall O vacancy concentration of about 2%. In systems with constant O vacancy concentrations, such as Ba1−xLaxCe0.9−xM0.1+xO2.95 and Ba1−xLaxCe0.85−xM0.15+xO2.925, structural parameters exhibited anomalous variations at La contents around x = 0.05. Possible factors that might lead to the occurrence of anomaly regions were considered: O 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.

Anomalous Variations of Unit Cell Parameters with Composition in Proton Conducting, ACeO3-Type Perovskite Solid Solutions. A.Kruth, G.C.Mather, J.R.Jurado, J.T.S.Irvine: Solid State Ionics, 2005, 176[7-8], 703-12