The Oxygen Potentials in the Y₂O₃-BaO-CuOx System

The oxygen potential for some chosen four and three-phase combinations has been measured at 750–1000°C using solid ZrO 2 +CaO electrolyte. On the basis of log P O 2 values, the Gibbs energies of reactions for many possible reactions in the system were obtained. The reactions with the Y 2 BaO 4 on the left sides exhibit negative entropy due to decomposition of Y 2 BaO 4 below about 850°C. The positive entropy for several reactions clearly indicates that ternary cuprates such as Y 2 BaCuO 5 , YBa 2 Cu 3 O x , YBa 4 Cu 3 O x and Y 3 Ba 8 Cu 5 O x are entropy-stabilized compounds. These cuprates become increasingly less stable with decreasing temperature and on cooling there are temperature limits below which these cuprates are metastable.