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

Abstract

The partial molar entropy of solution of oxygen in YBa₂Cu₃O_x, δs_O, has been computed using equilibrium data and the recently measured partial molar enthalpy of oxygen. While δh_O is independent of oxygen content (6 < x < 7), δs_O 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.