HIGH-TEMPERATURE PROTONIC CONDUCTORS WITH PEROVSKITE-RELATED STRUCTURES

The various perovskite-structured oxides found to be high-temperature protonic conductors are reviewed. They fall into two groups: the simple or ABO(3) types and the complex (or mixed) types of formula A(2)B'B''O-6 and A(3)B'B-2''O-9. Some microstructural features observed by TEM and electron diffraction are pointed out. A simple ''first-order'' model is then given to serve as a basis for consideration of various fundamental aspects of these protonic conductors. Three fundamental aspects are considered: the nature of proton incorporation by exposure to water vapor at high temperatures, the environment in which protons are located in the lattice, and the nature of proton migration. Proton incorporation is studied by water uptake experiments and the proton environment by spectroscopic techniques. Proton migration is studied by conductivity and isotope-effect experiments, which show that migration occurs by simple hopping of protons from one O2- ion to the next. However, the classical absolute rate theory (ART) requires modification because of the low efficiency of energy transfer between an excited proton and the cage of heavy ions that surrounds it.