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

Lai.85Bao.i5Cu04 exhibits high-7" c superconductivity with an onset temperature near 33 K as measured by resistivity and magnetic susceptibility. The nonsuperconducting end-member compound, La2CuC>4, which has an orthorhombic, Cmca, structure, exhibits semiconducting behavior. The orthorhombic distortion is proposed to result from a Peierls 2&F instability or a soft zone-boundary phonon mode. The role of Ba is to suppress the instability and stabilize a higher-symmetry tetragonal, I4/mmm, structure which is metallic and superconducting.

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The measurement of Cu K-edge x-ray absorption near-edge structure of monovalent, divalent, and trivalent copper oxides has been used to identify the presence of different valence of copper in the La2 -"(Sr,Ba) Cu04 (x 0.0-0.3) system. The results indicate the coexistence of Curt and CuIII states in these compounds. Theoretical calculations based on a cluster of Cu040qLagLaSr predict the presence of nearly degenerate electronic ground states representing nominally CuII and Cu III configurations.The observation of high superconducting transition temperatures in doped La2Cu04 (Refs. 1-5) has created new interest in the long disputed valency of Cu in these compounds. ' The compound La2Cu04 has an orthorhombic structure and exhibits semiconducting behavior at low temperatures.

Measurements of the transport critical-current density (Jc), magnetization Jc, and magnetoresistance in a number of bulk sintered samples of Y1Ba2Cu3Ox from several different laboratories indicate that the transport Jc is limited by weak-link regions between high Jc regions. The weak-link Jc has a Josephson character, decreasing by two orders of magnitude as the magnetic field is increased from 0.1 to 10 mT at 77 K. An examination of the grain-boundary region in Y1Ba2Cu3Ox shows no observable impurities or second phases to the scale of the [001] lattice planes (∼12 Å). The effect of intrinsic conduction anisotropy is discussed. A current-transfer model is proposed in which weak conduction along the c axis plays a role in limiting Jc at grain boundaries. Orienting the grains in the powder state during processing may result in enhanced transport Jc in bulk conductors.