The first report on proton conduction in perovskite-structured oxides dates back to 1981, when Iwahara and co-workers [18] showed that the perovskites SrCeO3 and BaCeO3 exhibit high proton-conductivities in hydrogen-containing atmospheres and may be applicable as electrolytic membrane in electrochemical cells such as fuel cells, steam electrolyzers, and gas sensors. Although BaCeO3 has remained a benchmark for high proton-conductivity in perovskites, many other related mate­rials have been developed and found to be proton conducting. Examples include more complex perovskites such as Ba3Caii8Nbi,82O873 [19—21], rare earth oxides such as Er2O3 [22], and pyrochlore-type oxides such as La2Zr2O7 [23, 24].