PHASE-EQUILIBRIA AT LOW OXYGEN PARTIAL-PRESSURE IN THE Y-BA-CU-O SYSTEM

The phase equilibria around YBa2Cu3O7-x (123) and YBa2Cu4O8 (124) phases at low oxygen partial pressure (less-than-or-equal-to 1 atm) were investigated by X-ray diffraction and thermal analysis. The coexistence of 123 and 124 phase was confirmed under 1 atm oxygen pressure. By using the high temperature X-ray diffraction method, the univariant reaction YBa2Cu3O7-x + Cu2O reversable Y2BaCuO5 + BaCu2O2 + O2 was identified. The oxygen partial pressure dependence of several univariant reactions has been investigated and the existence of two invariant reactions of L + O2 reversable YBa2Cu3O7-x + BaCuO2 + CuO + Cu2O and L + Y2BaCuO5 + O2 reversable YBa2Cu3O7-x + CuO + Cu2O was deduced to occur at 1103 K under 0.0032 atm O2 and at 1143 K under 0.0085 atm O2, respectively.