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: BaO₂ instead of BaO and YBa₂Cu₄O₈ instead of YBa₂Cu₃O_7-x. Two new phases have been observed: YBa₅Cu₂O_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 CuO₂-BaO-(½ Y + ½ Ba)O-BaO, while YBa₂Cu₂O_6-x appears to be isomorphous with the superconductor La_1.6Sr_0.4CaCu₂O₆. 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 YO_1.5-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, YO_1.5−2:1:1, 2:1:1-BaO₂, CuO-1:2:4 and 2:1:1-1:2:4.