Cation non-stoichiometry in yttrium-doped barium zirconate: phase behavior, microstructure, and proton conductivity

Abstract

Recent literature indicates that cation non-stoichiometry in proton-conducting perovskite oxides (ABO₃) can strongly influence their transport properties. Here we have investigated A-site non-stoichiometry in Ba_1−xZr_0.8Y_0.2O_3−δ, a candidate electrolyte material for fuel cell and other electrochemical applications. Synthesis is performed using a chemical solution approach in which the barium deficiency is precisely controlled. The perovskite phase is tolerant to barium deficiency up to x = 0.06 as revealed by X-ray diffraction analysis, but accommodates the non-stoichiometry by incorporation of yttrium on the A-site. The dopant partitioning can explain the decrease in cell constant with increasing x, the decrease in proton conductivity (the latter as measured by a.c. impedance spectroscopy under humidified atmosphere), and the decrease in grain size in the sintered compacts. Within the single-phase region barium deficiency also has a detrimental impact on grain boundary conductivity, as a result both of the decreased grain size, leading to a higher number density of grain boundaries and of an increased per boundary resistivity. At higher values of x, a two phase system is observed, with yttria appearing as the predominant secondary phase and the barium zirconate reverting to an undoped composition. From the relative concentrations of the observed phases and their lattice constants, the ternary phase behavior at 1600 °C (the sintering temperature) is generated. Both the bulk and grain boundary conductivities are sharply lower in the two-phase system than in the single phase compositions. The control over processing conditions demonstrates that it is possible to reproducibly prepare large-grained, stoichiometric BaZr_0.8Y_0.2O_3−δ with a total conductivity of 1 × 10⁻² Scm⁻¹ at 450 °C, while revealing the mechanisms by which barium deficiency degrades properties.