Thermodynamics of oxygen uptake for the YBa2Cu3Ox superconductor

Thermogravimetric analysis and pressure (p) monitoring in known volume systems are used to give phase diagram information (e.g. vant Hoff plots) for roughly 62Cu₃O_x. As an example we find for O₂ desorption for the important composition x = 6.8, beyond which superconductivity with transition temperature, T_s > 90 K becomes essentially independent of x, ΔH^o = 43.4 ±1 kcal/mo (O₂) and ΔS^o = 48.6 ±1 cal/Kmo (O₂). ΔS^o stays roughly constant for x > 6.7 but decreases for x < 6.7 indicating increased disorder on the O substructure. p (x) curves do not show phase separation which corresponds to above critical behavior, but given indications for limiting compositions near x = 6 and x = 7. Accordingly a model is proposed in which these compositions correspond to valencies of Cu⁺ and Cu³⁺ respectively for the symmetric Cu site (Ba coordinated) while the asymmetric Cu (Ba and Y coordinated) is taken to be 2+ over this whole range in x.