Atomistic Study of Doped BaCeO_3:  Dopant Site-Selectivity and Cation Nonstoichiometry

Rare earth (or yttrium) doped BaCeO₃ has been widely investigated as a proton conducting material. Usually, the trivalent dopants are assumed to occupy the Ce⁴⁺-site, which introduces oxygen vacancies into the perovskite structure and furthers the protonic conductivity. Recent studies indicate the possibility of dopant incorporation on the Ba²⁺-site, which is unfavorable for protonic conductivity. In this work atomistic simulation techniques, especially the supercell approach, have been developed to investigate the questions of dopant site-selectivity and cation nonstoichiometry in doped BaCeO₃. Our calculations predict that, on energetic grounds, Ba²⁺-site deficiency shifts trivalent dopant incorporation onto the Ba²⁺-site. These results confirm that the dopant partitioning or site-occupancy of trivalent dopants will be sensitive to the precise Ba/Ce ratio, and hence to the experimental processing conditions. The relative energies explain the experimentally observed "amphoteric" behavior of Nd with significant dopant partitioning over both Ba and Ce sites. Such partitioning reduces the concentration of oxygen vacancies, which, in turn, lowers proton uptake and decreases proton conductivity relative to dopant incorporation solely on the Ce⁴⁺ site