Thermochemistry of the Y₂O₃–BaO–Cu–O system

High temperature reaction calorimetry using molten lead borate as a solvent is used to study the thermochemistry of phases in the Y₂O₃–BaO–Cu–O system. Enthalpies of formation of YBa₂Cu₃O_x (6 < x < 7), YBa₂Cu₄O₈, Y₂BaCuO₅, Y₂Cu₂O₅, and BaCuO_2.01 phases have been determined. The thermodynamic relationships among these phases at room temperature have been assessed. The results show that the 123 phase is not thermodynamically stable with respect to assemblages containing combinations of Y₂BaCuO₅, Y₂Cu₂O₅, BaCuO₂, Y₂O₃, and CuO. The fully oxidized 123 phase is not stable with respect to the 124 and the above assemblages. Reactions at room temperature are shown as follows: YBa₂Cu₃O₇ (s) + CuO (s) → YBa₂Cu₄O₈ (s); δH = −31.9 ± 22.3 kJ mol⁻¹ and YBa₂Cu₃O_6.76 (s) → ½YBa₂Cu₄O₈ (s) + ¼Y₂BaCuO₅ (s) + ¾BaCuO_2.01 (s); δH = −78.9 ± 21.0 kJ mol⁻¹.