High-temperature protonic conduction in LaFeO₃-SrFeO _3-δ-SrZrO₃ solid solutions

The electrical conductivity of a pseudoternary system, consisting of LaFeO₃-SrFeO_3-δ-SrZrO₃, { La _{0.05 (1-x)} Sr_0.95+0.05x } (Zr_0.95 Fe _0.05)O_3-δ (x=0-0.5), was evaluated both in oxygen and hydrogen containing environments by a two-probe ac technique. Correlations of oxygen partial pressure, water vapor pressure, and composition dependences of the electrical conductivities with possible defect concentrations are examined by a semiquantitatively derived Brouwer diagram. In oxygen for x below 0.25, major carriers were identified to be oxide ions and electron holes, while for x=0.5 they were a mixture of oxide ions, electron holes, and protons. In a hydrogen containing environment, the oxides with compositions for x=0.25 and 0.5 were unstable. Thus, the electrical conductivity was analyzed in detail for x=0.1. It was found that the oxide conducts protons significantly at temperatures below 1273 K, which was evidenced by the isotope effect of hydrogen and deuteron on the electrical conductivity. The electrical conductivity of the oxide increased with decreasing oxygen partial pressure due to the variation of oxygen nonstoichiometry, accompanied by partial reduction of Fe ions in the oxide. The transport number of proton increased as the oxygen partial pressure decreased. The gas partial pressure and the composition dependences of the electrical conductivity and the transport number of proton could be well explained by the conceptual Brouwer diagram proposed in this work.