Single-crystal x-ray techniques were used to determine two structures that comprise the Y-Ba-Cu-O superconductor. A green phase has the orthorhombic ${\mathrm{Y}}_2$${\mathrm{BaCuO}}_5$ structure. An opaque phase (${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathrm{x}}$, 0≤x≤1.0) is tetragonal (a=3.859 Å, c=11.71 Å; space group P4¯m2;Z=1). The structure is related to a 1:1:3 AB${\mathrm{O}}_3$ perovskite: Ba and Y order in A sites and Cu is in B sites. Systematic oxygen vacancies between adjacent copper atoms lead to a fivefold-coordinate Cu in proximity to Y and a twofold-coordinate Cu in the vicinity of the Ba site.

• Received 10 March 1987

DOI:https://doi.org/10.1103/PhysRevB.35.7238