The 90-K superconducting phase of Y-Ba-Cu-O has been isolated, and its structure at room temperature determined by single-crystal x-ray diffraction. The material is an oxygen-defect perovskite, with composition ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{8\mathrm{-}\mathrm{x}}$. Entire planes of oxygen atoms between copper atoms are missing from the perovskite structure. The result is a quasi-two-dimensional structure in which sandwiches of ${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{8\mathrm{-}\mathrm{y}}$ are separated by layers of Y.

• Received 13 March 1987

DOI:https://doi.org/10.1103/PhysRevB.35.7245