Defect chemistry and transport properties of Ba_xCe_0.85M_0.15O_3-δ

The site-incorporation mechanism of M³⁺ dopants into A²⁺B⁴⁺O₃ perovskites controls the overall defect chemistry and thus their transport properties. For charge-balance reasons, incorporation onto the A²⁺-site would require the creation of negatively charged point defects (such as cation vacancies), whereas incorporation onto the B⁴⁺-site is accompanied by the generation of positively charged defects, typically oxygen vacancies. Oxygen-vacancy content, in turn, is relevant to proton-conducting oxides in which protons are introduced via the dissolution of hydroxyl ions at vacant oxygen sites. We propose 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 can exhibit dopant partitioning as a consequence of barium evaporation at elevated temperatures. Such partitioning and the presence of significant dopant concentrations on the A-site negatively impact proton conductivity. Specific materials examined are Ba_xCe_0.85M_0.15O_3-δ (x = 0.85 - 1.20; M = Nd, Gd, Yb). The compositional limits for the maximum A-site incorporation are experimentally determined to be: (Ba_0.919Nd_0.081)(Ce_0.919Nd_0.081)O₃, (Ba_0.974Gd_0.026)(Ce_0.872Gd_0.128)O_2.875, and Ba(Ce_0.85Yb_0.15)O_2.925. As a consequence of the greater ability of larger cations to exist on the Ba site, the H₂O adsorption and proton conductivities of large-cation doped barium cerates are lower than those of small-cation doped analogs.