Quaternary phase relations near YBa 2Cu 3O 6+ x at 850°C in reduced oxygen pressures

Abstract

The quatenary phase relations near YBa 2Cu 3O 6+ x at 850°C were determined by oxygen coulometric titration and microanalysis. The phase relations change markedly as the oxygen pressure decreases, due primarily to the formation of a liquid phase and a reduced oxide, BaCu 2O 2. Nine invariant reactions were discovered, including decomposition of YBa 2Cu 3O 6+ x into Y 2BaCuO 5, BaCu 2O 2 and YBa 3Cu 2O 6+ y . The lower limit of oxygen partial pressure at which pure YBa 2Cu 3O 6+ x is stable is 4.0×10 −4 atm O 2 at 850°C; the lower limit increases in the presence of BaCuO 2 and Cu 2O impurities. These results indicate that the temperatures used for single crystal growth of YBa 2Cu 3O 6+ x could be lowered by using reduced oxygen pressures or, alternatively, that constant temperature crystal growth could be induced by increasing the oxygen partial pressure.