Variation of the partial thermodynamic properties of oxygen with composition in YBa₂Cu₃O_7−δ

The variation of equilibrium oxygen potential with oxygen concentration in YBa₂Cu₃O_7−δ has been measured in the temperature range of 773 to 1223 K. For temperatures up to 1073 K, the oxygen content of the YBa₂Cu₃O_7−δ sample, held in a stabilized-zirconia crucible, was altered by coulometric titration. The compound was in contact with the electrolyte, permitting direct exchange of oxygen ions. For measurements above 1073 K, the oxide was contained in a magnesia crucible placed inside a closed silica tube. The oxygen potential in the gas phase above the 123 compound was controlled and measured by a solid-state cell based on yttria-stabilized zirconia, which served both as a pump and sensor. Pure oxygen at a pressure of 1.01 × 10⁵ Pa was used as the reference electrode. The oxygen pressure over the sample was varied from 10^-1 to 10⁵ Pa. The oxygen concentrations of the sample equilibrated with pure oxygen at 1.01 × 10⁵ Pa at different temperatures were determined after quenching in liquid nitrogen by hydrogen reduction at 1223 K. The plot of chemical potential of oxygen as a function of oxygen non-stoichiometry shows an inflexion at δ ∼ 0.375 at 873 K. Data at 773 K indicate tendency for phase separation at lower temperatures. The partial enthalpy and entropy of oxygen derived from the temperature dependence of electromotive force (emf ) exhibit variation with composition. The partial enthalpy for °= 0.3, 0.4, and 0.5 also appears to be temperature dependent. The results are discussed in comparison with the data reported in the literature. An expression for the integral free energy of formation of YBa₂Cu₃O_6.5 is evaluated based on measurements reported in the literature. By integration of the partial Gibbs’ energy of oxygen obtained in this study, the variation of integral property with oxygen concentration is obtained at 873 K