Since the discovery of superconductivity above 30 K by Bednorz and M\"uller in the La copper oxide system, the critical temperature has been raised to 90 K in Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and to 110 and 125 K in Bi-based and Tl-based copper oxides, respectively. In the two years since this Nobel-prize-winning discovery, a large number of electronic structure calculations have been carried out as a first step in understanding the electronic properties of these materials. In this paper these calculations (mostly of the density-functional type) are gathered and reviewed, and their results are compared with the relevant experimental data. The picture that emerges is one in which the important electronic states are dominated by the copper $d$ and oxygen $p$ orbitals, with strong hybridization between them. Photon, electron, and positron spectroscopies provide important information about the electronic states, and comparison with electronic structure calculations indicates that, while many features can be interpreted in terms of existing calculations, self-energy corrections ("correlations") are important for a more detailed understanding. The antiferromagnetism that occurs in some regions of the phase diagram poses a particularly challenging problem for any detailed theory. The study of structural stability, lattice dynamics, and electron-phonon coupling in the copper oxides is also discussed. Finally, a brief review is given of the attempts so far to identify interaction constants appropriate for a model Hamiltonian treatment of many-body interactions in these materials.

Electronic structure of the high-temperature oxide superconductors

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’’ CuO2 layers in the a‐b planes loosely bonded by one‐dimensional fencelike square‐planar CuO3 chains in the b‐c planes.

Structure of the single‐phase high‐temperature superconductor YBa2Cu3O7−δ

One-dimensional coordination polymers: applications to material science

Probably the most unusual crystal-chemical aspect of the new high-temperature oxide superconductors is their significant variability in oxygen content. For the YBa2Cu3O7–δ family of compounds, the oxygen stoichiometry varies in the range 0 < δ < 1 under normal synthetic conditions, with the optimal 91-K superconductive transitions obtained near δ=0 (see, for example, ref. 1). Work presented so far on the dependence of both superconducting and normal-state properties on oxygen stoichiometry has considered only the value of δ in YBa2Cu3O7–δ. Here we present data that indicate the issue to be considerably more interesting and complex, and propose that the microscopic variables thus far not considered include the distribution of oxygen over the available sets of sites in the structure, with that distribution strongly dependent on synthetic conditions. In addition to the dependence of the bulk transition temperture (Tc), normal-state resistivity and crystallographic symmetry on the microscopic distribution, we find the magnitude of the magnetic susceptibility in the normal state to be strongly affected by the microscopic state of the oxygen subsystem. We also report the existence of a single-phase 60-K bulk superconductor at stoichiometry YBa2Cu3O6.6–6.7, which we postulate to have an ordered array of oxygen vacancies, and two distinct plateau regions in the variation of Tc with x which are either unobserved or only hinted at under other synthetic conditions.

Oxygen stoichiometry, superconductivity and normal-state properties of YBa2Cu3O7-δ

The capability of sodium hydride as a reducing agent in oxide deintercalation reactions is explored. The Ni(III) perovskite LaNiO3 can be reduced topotactically to LaNiO2, isostructural with the “infinite layer” cuprates, using solid sodium hydride in a sealed evacuated tube at 190 ≤ T/°C ≤ 210, and a similar infinite-layer phase is prepared by reduction of NdNiO3. Structural characterization indicates the coexistence of incompletely reduced regions, with five-coordinate Ni centers due to the introduction of oxide anions between the NiO23- sheets, giving samples with a refined stoichiometry of LaNiO2.025(3). Neutron powder diffraction and magnetization measurements indicate that the lamellar Ni(I) phase does not show the long-range antiferromagnetic ordering characteristic of isoelectronic Cu(II) oxides. This may be due either to the influence of the interlamellar oxide defect regions or to the reduced covalent mixing of Ni 3d and O 2p levels.

Sodium Hydride as a Powerful Reducing Agent for Topotactic Oxide Deintercalation: Synthesis and Characterization of the Nickel(I) Oxide LaNiO2

Materials within the phase diagram Y2O3–BaO–CuO have recently generated great interest because of the observation of superconductivity with a critical temperature (Tc ) of ≳90 K1,2; the current consensus is that YBa2Cu3O7−x (x ≈ 0.2) is the superconducting phase (see, for example ref. 3). Hazen et al.4 concluded, from a single-crystal X-ray diffraction experiment, that its structure is based on the corner-linked octahedral perovskite structure: the tetragonal unit cell (space group P&4bar;m2) arises from a tripling of the c-axis resulting from the ordering of yttrium and barium cations and associated oxygen defects. Here we present high-resolution neutron powder diffraction data for this phase with x = 0.15(7). Our results agree with ref. 4 with respect to cation positions, but the location of one set of oxygen defects is substantially different and necessitates the lowering of lattice symmetry from tetragonal to orthorhombic (space group Pmmm), in agreement with separate findings5–8. In contrast to the 35-K superconductor, La1.85Ba0.15CuO4, the corner-linked CuO4 planar groups are connected not only as sheets in the a–b plane but also as chains parallel to the ft-axis. The average copper valence is 2.23 (5). From bond valence arguments we infer that the Cu2+ and Cu3+ ions preferentially occupy square pyramidal and square planar sites respectively. Crystal chemical considerations suggest that these structural features may be involved in the superconducting mechanism.

Structure and crystal chemistry of the high-Tc superconductor YBa2Cu3O7−x

We have determined the phase diagram of the Y‐Ba‐Cu‐O system through structural, superconducting critical temperature and critical current density characterization. Our results show that a single‐phase compound with a stoichiometry YBa2Cu3Oy is responsible for the high‐temperature superconductivity (92.5 K) in this system.

J. D. Grace

Papers

Structure of the single‐phase high‐temperature superconductor YBa2Cu3O7−δ

Structure and crystal chemistry of the high-Tc superconductor YBa2Cu3O7−x

Phase diagram and superconductivity in the Y‐Ba‐Cu‐O system

La2−xSrxCuO4 and YBa2Cu3O6.5: New high Tc superconducting oxides