Reexamination of the phase diagram of the BaO-ZrO2-Y2O3 system: investigation of the presence of separate region in Y-doped BaZrO3 solid solution and the dissolution of Zr in Ba3Y4O9.

Y-doped BaZrO3 (BZY) is a promising candidate for electrolytes in protonic ceramic fuel cells. To achieve the high proton conductivities of sintered electrolyte membranes, the poor sintering nature is one of the problems because of the high resistance at grain boundaries. To obtain single-phase BZY with sufficient grain growth (> 1 μm), the information of phase stable conditions of BZY is important. However, in previous studies, there are discrepancies in the single-phase composition range of BZY, and the phase relationships between BZY and other phases at a sintering temperature (1600 °C). In this work, we clarified the phase stable conditions of BZY by reexamining the critical part of the phase diagram of the BaO-ZrO2-Y2O3 system. Based on X-ray diffraction, at certain temperatures below 1600 °C, the single-phase composition range of BZY is not continuous at ~ 10% in the molar ratio of Y/(Zr + Y). Even when sintering at 1600 °C, this separate region probably retards the formation of single-phase and grain growth. Besides, we revealed the other ternary phases, i.e., γ phase and Zr-substituted Ba3Y4O9 are stable in XBaO < 50 mol% and can coexist with BZY at 1600 °C. This information is discussed on the Gibbs triangles and should be useful for all researchers working on BZY. [ABSTRACT FROM AUTHOR]