Phase equilibria among the superconductors in the Y2O3–BaO–Cu–O system

The enthalpy of formation of Y2Ba4Cu7O14.864 at room temperature has been determined by using high-temperature reaction calorimetry. The standard enthalpy of formation of the 247 phase is ΔH°f = −5463 ± 18 kJ mol−1. Phase relationships among the superconductors, namely, the 123, 124, and 247, are assessed. It is intrinsic that impurity phases, such as Y2Cu2O5, BaCuO2, CuO, etc., coexist with the superconductors at equilibrium. Equilibria among the superconductor phases in the Y2O3–BaO–Cu–O system have been determined. The results show that, at the 124 bulk composition, the 247 phase coexists with the excess CuO in a narrow area of pO2 ∼ T space confined by the stability fields of the 124 phase and a mixture of the 123 + CuO. The standard free energies of carbonation and hydration of the 247 phase are also determined, which are −629 ± 13 kJ mol−1 and −873 ± 45 kJ mol−1, respectively. Like the 123 and 124 phases, the 247 phase is thermodynamically unstable for corrosion by water vapor and carbon dioxide in ambient atmosphere.