PHASE-DIAGRAM AND NEW PHASES IN THE Y-BA-CU-O SYSTEM AT HIGH OXYGEN-PRESSURE

The Y-Ba-Cu-O phase diagram has been determined at 980°C at high oxygen pressure 200 bar=20 mPa. It shows several differences from the phase diagram at 1 bar. Highly oxidized phases are stabilized: BaO2 instead of BaO and YBa2Cu4O8 instead of YBa2Cu3O7-x. Two new phases have been observed: YBa5Cu2O10-x has a tetragonal unit cell with a=5.88 Å and c=8.04 Å, consistent with a four layered structure such as CuO2-BaO-( 1 2Y + 1 2Ba)O-BaO, while YBa2Cu2O6-x appears to be isomorphous with the superconductor La1.6Sr0.4CaCu2O6. In addition, we find a high-pressure BaCuO*x phase, its X-ray diffraction (XRD) pattern indicates a perovskite related layered structure which has a unit cell a=5.68 Å, c=14.40 Å. The formation of compounds on the YO1.5-BaO line: Y2Ba2O5, Y2BaO4 and Y2Ba4O7 is suppressed at high P(O2). Tie lines are observed to join CuO-2:1:1, 2:0:2-2:1:1, YO1.5-2:1:1, 2:1:1-BaO2, CuO-1:2:4 and 2:1:1-1:2:4. © 1990.