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 YBa2Cu3Ox (6 < x < 7), YBa2Cu4O8 Y2BaCuO5 Y2Cu2O5 and BaCuO2.01 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 Y2Cu2O 5 BaCuO2 Y2O 3 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: YBa2Cu307 (s) + CuO (s) → YBa2Cu408 (s); ΔH = -31.9 ± 22.3 kJ mol-1 and YBa2Cu3O 6.76 (s) — 1/2 YBa2Cu4O8 (s) + 1/4Y2BaCuO5 (s) + 3/4BaCuO2.01 (s); ΔH = -78.9 ± 21.0 kJ mol-1.