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 Y2BaCuO5 structure. An opaque phase (YBa2Cu3O6+x, 0x1.0) is tetragonal (a=3.859 A, c=11.71 A; space group P4m2;Z=1). The structure is related to a 1:1:3 ABO3 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.
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