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

The enthalpy of formation of Y₂Ba₄Cu₇O_14.864 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 p_O(2) to approximately 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.