Influence of Doping on the Structural Transformations of the Proton Conducting Perovskite BaCe1xYxO3-D

From neutron diffraction it is known that the BaCeO₃ perovskite undergoes a sequence of phase transformations from high temperature cubic C to rhombohedral R, to orthorhombic O1 (Imma) and to orthorhombic O2 (Pnma). Doping Y³⁺ on the Ce⁴⁺ site introduces charge compensating O vacancies (V_O) that may be partially filled with OH complexes with exposition to H₂O, so making the material an ionic conductor. Anelastic relaxation experiments have been carried out on samples doped with 2%Y and 10%Y; the real part s’(T) of the complex elastic compliance presents softenings at the transitions, and the loss s’’/s’ curves allow the content of V_O and H to be monitored. Doping has a strong effect on the temperature of the Pnma/R transition: with 10%Y in the fully hydrated state T_O1-R increases up to 750 K while after full outgassing falls to 500 K, meaning that the introduction of ~5% V_O shifts the transition of 250 K. While the effect of cation substitution on the transitions temperature is easily explained in terms of simple arguments usually valid for perovskites based on bond length considerations, the remarkable stabilization of the R phase by V_O requires to take into account the anomalous sequence of phase transitions of undoped BaCeO₃, where the R structure transforms into orthorhombic Pnma on cooling with the loss of an octahedral tilt system.