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We have determined the crystal structure of the single-phase stoichiometric high-temperature superconductor in the Y-Ba-Cu-O system using high-resolution neutron powder diffraction. This compound has an orthorhombic structure with space group Pmmm and lattice constants a = 3.8231 A, b = 3.8864 A, and c = 11.6807 A. The structure consists of "dimpled" Cn0 2 layers in the a-b planes loosely bonded by one-dimensional fenceIike square-planar cun, chains in the b-c planes. Recently, superconductivity above 90 K was reported by Wu et at. l in a multiphase sample with a nominal composition of V!.2 B8.,,).8 0.1 2 0 4 _ b' Subsequent studies by several groups2-S identified the superconducting compound as VEa 2 Cu} Oy. Although the metal-ion stoichiometry is wen established the structure is still in question. One of the simplest ordered perovskite structures with a YBa 2 CU3 Oy stoichiometry consists of 1: l:n stacking of cubic perovskite building blocks with Cu atoms on the cube corners, Y or Ba atoms in the center of the cube, and 0 atoms on the cube edges. From powder x-ray diffraction data Cava et al. 3 pro

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