It was recalled that the site-incorporation mechanism of M3+ dopants in A2+B4+O3 perovskites controlled the overall defect chemistry and their transport properties. To maintain charge balance, incorporation onto the A2+-site required the creation of negatively charged point defects such as cation vacancies. Incorporation into the B4+-site was accompanied by the generation of positively charged defects; typically O vacancies. The O-vacancy content was important to proton-conducting oxides, where protons were introduced via the dissolution of hydroxyl ions at vacant O sites. It was proposed here, on the basis of X-ray powder diffraction studies, electron microscopy, chemical analysis, thermal gravimetric analysis and alternating-current impedance spectroscopy, that nominally B-site doped barium cerate could exhibit dopant partitioning as a consequence of Ba evaporation at elevated temperatures. Such partitioning and the presence of significant dopant concentrations on the A-site negatively affected proton conductivity. Specifically examined were BaxCe0.85M0.15O3-δ (x = 0.85 to 1.20; M = Nd, Gd, Yb). The compositional limits for maximum A-site incorporation were experimentally determined to be: (Ba0.919Nd0.081) (Ce0.919Nd0.081) O3, (Ba0.974Gd0.026) (Ce0.872Gd0.128) O2.875 and Ba(Ce0.85Yb0.15) O2.925. As a consequence of the greater ability of larger cations to exist on Ba sites, the H2O adsorption and proton conductivities of large-cation doped barium cerates were lower than those of small-cation doped analogues.

Defect Chemistry and Transport Properties of BaxCe0.85M0.15O3-δ. J.Wu, L.P.Li, W.T.P.Espinosa, S.M.Haile: Journal of Materials Research, 2004, 19[8], 2366-76