Phase Diagram of YBa2Cu3O7-x, Y2Ba4Cu7O15-x and YBa2Cu4O8 Superconductors

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Abstract
The phase diagrams of Y-system superconductors with compositions of 123 and 124 are given as a function of temperature and partial pressure of oxygen over the range 700 to 1000°C and 0.5≦P(O2)≦10 atm. The stable regions of the phases YBa2Cu3O7-x (123), Y2Ba4Cu7O15-x (247) and YBa2Cu4O8(124) were determined. The phase boundaries of 247/124 (starting composition of 124) and 123/124 (starting composition of 123) were found to be represented by the following equations, respectively: log P(O2)(atm)=(7.89±0.08)-(0.86±0.01)×104/T(K) log P(O2)(atm)=(12.1±1.6)-(1.25±0.18)×104/T(K). The 123 phase is stable only in its semiconductor phase (tetragonal phase) and the superconducting phase (orthorhombic phase) is metastable at all temperatures. The 124 phase is more stable than the 123 orthorhombic phase.