As a result of our P-T-x phase diagram studies under high oxygen pressure in the Y-Ba-Cu-O system, we have discovered two bulk superconducting phases YBa2Cu3.5O7 + x(Tc≈ 40 K) and YBa2Cu4O8 + x(Tc≈ 80 K) and have roughly determined their thermodynamic fields of stability. Both phases are metastable under normal conditions and can be synthesized only under high oxygen pressure. YBa2Cu4O8 + xhas been observed in the past by HREM as a defect in "123" decomposed powders and as an ordered phase coexisting in "123" films. The approximate conditions of thermodynamic stability of the "124" phase have been determined in the course of our P-T-x studies, so that it was possible to synthesize bulk quantities of the material. We present here a thermodynamic, structural and physical characterization of these compounds. Although it is metastable under normal pressure, the "124" phase shows a high thermal stability of oxygen up to 850 °C. © 1989.
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