Phase diagram and new phases in the Y-Ba-Cu-O system at high oxygen pressure

Abstract

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: BaO 2 instead of BaO and YBa 2Cu 4O 8 instead of YBa 2Cu 3O 7− x . Two new phases have been observed: YBa 5Cu 2O 10− x has a tetragonal unit cell with a=5.88 Å and c=8.04 Å, consistent with a four layered structure such as CuO 2-BaO-( 1 2 Y + 1 2 Ba)O-BaO, while YBa 2Cu 2O 6− x appears to be isomorphous with the superconductor La 1.6Sr 0.4CaCu 2O 6. 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 YO 1.5-BaO line: Y 2Ba 2O 5, Y 2BaO 4 and Y 2Ba 4O 7 is suppressed at high P(O 2). Tie lines are observed to join CuO-2:1:1, 2:0:2-2:1:1, YO 1.5-2:1:1, 2:1:1-BaO 2, CuO-1:2:4 and 2:1:1-1:2:4.