OXYGEN PARTIAL-PRESSURE DEPENDENCE OF THE YTTRIUM SOLUBILITY IN Y-BA-CU-O SOLUTION

The ytrrium solubility in Ba-Cu-O solvent with a Ba to Cu ratio of 3 t o 5 was investigated under different oxygen partial pressure [P(O-2) = 2, 21, 100%]. A small amount of the solution was taken out by dipping thermally equilibrated MgO single crystals, and compositions of these specimens were analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES). The measurements were performed for the sampl es prepared in the temperature range from approximately 950 degrees C up to 1100 degrees C. The peritectic temperature of YBa2Cu3O7-x (Y123) decreased with decreasing oxygen partial pressure. On the other hand, the Y123 liquidus lines do not show remarkable oxygen partial pressur e dependency. Accordingly, the yttrium solubility at the peritectic te mperature of Y123 under P(O-2) = 100% was larger than those for the ot her conditions [P(O-2) = 2% and 21%]. Assuming a regular solution, exp ressions for the solubility and the enthalpy of dissolution of Y123 an d Y2BaCuO5 (Y211) were derived from classical thermodynamic calculatio ns and found to be 289 kJ/mol at P(O-2) = 0.02, 239 kJ/mol at P(O-2) = 0.21 atm, 206 kJ/mol at P(O-2) = 1 atm for Y123, and 105 kJ/mol at P( O-2) = 0.02 atm, 88.0 kJ/mol at P(O-2) = 0.21 atm, and 88.4 kJ/mol at P(O-2) = 1 atm for Y211. Furthermore, the Jackson alpha factor of Y123 was estimated to be about 20, confirming a faceted growth nature of t his crystal.