Indium doping in barium cerate: the relation between local symmetry and the formation and mobility of protonic defects

The solid solution series Ba(In,Ce)O3-ä has been investigated with respect to structure, formation, and mobility of protonic defects. Compared to the limited solubility of Y2O3 in BaCeO3 and BaZrO3, the complete solubility of In2O3 is suggested to reflect a relation between absolute hardness of the dopant and the ease of insertion into the hosting lattices. Extended X-ray absorption fine structure (EXAFS) was used to probe the local environment of In3+ in barium cerate: in the surroundings of the dopant, the orthorhombic structure is strongly modified, resulting in an increase of local symmetry. The InO6 octahedra are very regular, and there is no indication for any defect clustering. This is suggested to be the main reason for the low entropy of formation of protonic defects by water dissolution. The mobility of such defects is slightly lower than in Y-doped BaCeO3, but at high dopant levels the high local symmetry allows for formation of very high concentrations of protonic defects. This leads to high proton conductivities, which render In3+ an attractive dopant for BaCeO3-based proton conductors.