Direct calorimetric determination of energetics of oxygen in YBa₂Cu₃O_x

The enthalpy of oxidation of YBa₂Cu₃O_x (5.97 ≤ x ≤ 6.94) was measured directly by high-temperature reaction calorimetry. The partial molar enthalpy of oxygen dissolving in the structure is constant at -237 ± 5 kJ/mole O₂ over the whole composition range. No change in Δh_O2 is observed at either the tetragonal/orthorhomic transition or at x = 6.5, the composition in which all copper could be divalent. This behavior supports structural models in which the change in speciation from x = 6 to x = 7 is very gradual, with Cu⁺ and Cu³⁺ (or their electronic equivalents in the oxygen p-band) coexisting in concentrations which vary smoothly with composition. It is also consistent with models which consider the solid solution series to be a complex intergrowth of microdomains of oxidized and reduced end members. The observations rule out that the composition YBa₂Cu₃O_6.5 is any sort of "titration endpoint" with very different defect chemistry on either side.