Structural transitions and proton conduction in nonstoichiometric A3B'B2 double-prime O9 perovskite-type oxides

In the authors' recent studies, the complex perovskite-type oxides of the formula A3Ca1+xNb2-xO9-δ, where A = Ba, Sr, have been shown to become excellent protonic conductors (when x > 0) after exposure to water vapor. In this work, the authors further explore the structure-properties correlation of these compounds. Using X-ray diffraction, TEM, and EDX microanalysis, it is shown that the 1:2 ordering, present in the stoichiometric samples, is gradually replaced by 1:1 order and random order. The differently ordered regions appear as nanoscale fluctuations. Comparison of the protonic conductivity reveals that a steep increase in the effective mobility, μH, of protons accompanies the structural changes. This behavior can be interpreted by the concept of percolation in composite media. In addition, the change in structure with composition gives rise to striking changes in mechanical properties