Oxygen Nonstoichiometry of (Nd2/3Ce1/3)4(Ba2/3Nd1/3)4Cu6O16+x

The equilibrium oxygen content of (Nd2/3Ce1/3)4(Ba2/3 Nd1/3)4Cu6O16+x was measured by coulomeric titration in the temperature range 550-820°C and at oxygen pressures between 10-3 and 1 atm. From the results, the partial molar enthalpy and entropy of oxygen were calculated as functions of the oxygen content. The data were explained based on a point defects model and the equilibrium constants of the redox and charge disproportionation reactions involved were derived. A phase transition near the point x ≈ 1.83, which nominally corresponds to one trivalent copper cation per elementary unit, was observed.