The enthalpy of oxidation of YBa2Cu3Ox (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 ΔhO2 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 YBa2Cu3O6.5 is any sort of "titration endpoint" with very different defect chemistry on either side.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Condensed Matter Physics
- Physical and Theoretical Chemistry
- Inorganic Chemistry
- Materials Chemistry