The crystal structures and phase transitions in Y-doped BaCeO3: their dependence on Y concentration and hydrogen doping

The crystal structures and phase transitions in BaCe1−xYxO3−δ (0≤x≤0.3) that occur in response to heat treatment in oxidizing, reducing, and water-vapor-containing atmospheres were determined by neutron powder diffraction at room temperature. Single-phase BaCe1−xYxO3−δ (δ≅x/2) can be synthesized by O2-annealing samples prepared by a solid-state reaction method, wherein they crystallize into perovskite-type structures (orthorhombic Pmcn for x≤0.1 and rhombohedral R3̄c for x≥0.15). Within the composition range of 0.15≤x≤0.25, where maximum total conductivity is observed, the rhombohedral structure converts partially to a monoclinic I2/m structure upon exposure to water- or H2-containing atmospheres (including laboratory air) at elevated temperatures. This study may suggest a correlation between the crystal structures, particularly the I2/m phase, and the proton transport properties.