Thermodynamics of Y₂Cu₂O₅ and YCuO₂ and Phase Equilibria in the Ba–Y–Cu–O System

Electromotive force (EMF) measurements using ZrO2 solid electrolyte were carried out in the Ba–Y–Cu–O system. Gibbs free energies for the reaction in the cell electrodes were summarized in the equations with linear temperature dependency. The standard free energies of the formation of Y2Cu2O5 and YCuO2 were derived from EMF data and compared with published informations on the stability of Y2Cu2O5.From the results of the X-ray diffraction measurements for the quenched specimens, the phase equilibria in Y–Cu–O system were determined in the temperature range 923 K to 1223 K. Based on the experimental results, YCuO2 was stable above 1115 K and at the oxygen partial pressure higher than 7.18×10−4 Pa. In the quaternary Ba–Y–Cu–O system, only a few cell could show the stable EMF. The solubilities of barium in both Y2Cu2O5 and YCuO2 were negligibly small.