Conversion of 124 into 123+CuO and 124, 123 and 247 phase regions in the Y- Ba-Cu-O system

The phase diagram of the Y-Ba-Cu-O system is given as a function of temperature and partial pressure of oxygen over the range between 500°C and 1000°C and 10^-6<P(O₂)<10² bar. Conversion of 124 into 123+CuO is interesting because it can produce nonsuperconducting CuO islands and highly strained local regions, both of which may act as flux pinning centers. The partial pressure of oxygen necessary for conversion from YBa₂Cu₄O₈ (124) to YBa₂Cu₃O₇ (123) decreases with decreasing temperatures, and below 850°C the boundary between 123 and 124 phase regions is found to fall approximately as logP(O₂)=19.3-22000/T. The 123 decomposition boundary is known to lie near logP(O₂)=10.2-14000/T. Since these two curves interest at about 600°C, the 123 phase is unstable at lower temperatures for all P(O₂), and so 123 cannot be synthesized near thermodynamic equilibrium at low temperatures.