Thermodynamic stability field of the 123 and 124 phases in the Y2O3--BaO--Cu--O system

The partial molar entropy of solution of oxygen in YBa2Cu3Ox,Δ bar sO, has been computed using equilibrium data and the recently measured partial molar enthalpy of oxygen. While Δ bar h0 is independent of oxygen content (6.0<x<7), Δ bar sO depends on x. The pO2, T location of phase equilibrium between the 123 and 124 superconducting phases is calculated. The standard free energies of interactions of the superconducting phases with carbon dioxide (CO2) and water vapor (H2O) have been assessed. The free energies of carbonation of the fully oxidized 123 and 124 phases are -323±22 kJ mol-1 and -291±32 kJ mol-1, respectively. The free energies of hydration of the 123 and 124 are -397±22 kJ mol-1 and -469±32 kJ mol-1, respectively. These large exothermic values show that degradation of these superconductors at ambient conditions is thermodynamically favorable