Effect of oxygen partial pressure on the lower solubility limit of Nd1+xBa2−xCu3O7

The lower solubility limits of Nd_1+xBa_2−xCu₃O₇ (Nd123ss) in 1 bar, 0.2 bar, 0.01 bar, and 0.001 bar oxygen partial pressure (P_O2) were studed by differential thermal analysis (DTA) and X-ray diffraction (XRD). Large relative changes in stability with decreasing P_O2 explain why sharp superconducting transitions can be achieved in P_O2 ≤ 0.01 bar while processing in 0.2 to 1 bar P_O2 tends to produce broad transitions. While the lower solubility limits in 0.01 bar and 0.001 bar P_O2 remain at x = 0.00, the solubility limits in 0.2 bar and 1 bar P_O2 show a narrowing with decreasing temperature. Because of the narrowing of the solubility range in 1 bar P_O2, oxygen annealing of Nd123 initially processed in low P_O2 will result in precipitation of second phases which act as flux pinning centers.