Phase relations for the Y-Ba-Cu-O system near 1:2:3 stoichiometric compositions at high temperatures have been studied through microstructural observation. It has been found that YBa2Cu3Ox decomposes into a Y2BaCuO5(211) plus liquid (a mixture of BaCuO2 and CuO) phase at around 1050°C and the 211 phase further decomposes into Y2O3 plus liquid at around 1200°C. Even at 1500°C Y2O3 seems to be stable. On cooling Y2O3 reacts with the liquid to produce the 211 phase by peritectic solidification. At around 1000°C the 211 phase reacts with the liquid to produce the YBa2Cu3Ox phase again by the peritectic reaction.
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