Subsolidus phase relationships of the BaO-R2O3-CuOz (R = Eu, Dy, and Ho) systems under carbonate-free conditions at T = 810 °C and pO2=100Pa

For applications of phase equilibria to second-generation coated-conductor high-Tc superconductor processing, phase diagrams constructed under carbonate-free conditions are needed. Using a procedure for preparing carbonate-free precursors based on BaO, the phase diagrams of three BaO-R2O3-CuOz systems (R = Eu, Dy, and Ho) were determined at 810 °C and pO2=100Pa (0.1% O2 by volume). The three phase diagrams are substantially different from one another. Among the three systems, Eu3+ has the largest ionic radius and therefore most closely matches the ionic size of Ba2+. Two solid solution series, namely, Ba2−xEuxCuO3+z and Ba2−xEu1+xCu3O6+z were found in the BaO-Eu2O3-CuOz system. The BaO-Dy2O3-CuOz and the BaO-Ho2O3-CuOz systems do not contain solid solution phases. In these systems, which contain the smaller R = Dy, and Ho rare earths, the Ba2−xEuxCuO3+z solid solution and Eu2CuO4 phases of the Eu-system are replaced by the Ba6RCu3Oz (6 1 3) phase and the R2Cu2O5 phase, respectively. Additionally, the Ba4RCu3Oz (4 1 3) phase appears in systems with smaller R (Dy and Ho) but not in the Eu-system. The Ho-system contains the Ba3R4O9 phase, as well. Consequently, the tie-line relationships are substantially different in these three systems. Among the three diagrams, the diagram for R = Ho is most similar to that of the R = Y system.