Copper oxide superconductors structure

In early 1987, the composition and structure of the La-Ba-Cu -O superconductor was still unknown to the general public in the United States. By March of that year certain facts became known from Japanese publications. But at this point in time, a newer, higher Tc (> 90 K) material was announced. This new copper oxide superconductor was quite easy to prepare and, in addition to interested physicists, these new materials could be synthesized by ceramists, chemists, metallurgists, material scientists, or anyone with a knowledge of a chemical approach to solid-state materials. Even high school students developed simple methods for the synthesis of these compounds. The "high" transition temperature and the possible use of liquid nitrogen made research in superconductivity accessible to most scientists and laboratories. The media also capitalized on this worthy news report and published it in newspapers and also presented it on television as a news item. 

The intent of this chapter is twofold. One is to give brief structural descriptions of many of the copper-oxide superconductors. For in-depth information, the reader is referred to the original publications. The second is to provide detailed crystallographic information (lattice constants, positional and thermal parameters, space groups, etc.), and compositional data on many of the superconductors discussed. Also, calculated x-ray powder diffraction patterns for these same compounds are tabulated. It is hoped that such information will prove useful to the superconductivity researcher. 

A common feature of all the new ceramic superconductors is that they are cuprates, that is, they are complex copper oxides. The structure of YBCO is given in Fig. 19.3, which also shows that it is related to the perovskite structure (Fig. 4.17). Synthesis of YBCO is remarkably easy appropriate amounts of dry yttrium oxide (Y203), copper oxide (CuO), and barium carbonate (BaC03) are ground together into a fine, well-mixed... 

Cava, R.J. Structural Chemistry and the Local Charge Picture of Copper Oxide Superconductors, Science, 656 (February 9, 1990). 

All the copper oxide superconductors ever known contain a common structural unit the two-dimensional Q1O2 sheet... 

Pale yellow cerium dioxide (ceria, ceric oxide) has the fluorite structure and is used in catalysis" ", solid oxide fuel cells (SOFC)", thin film optical waveguides" , reversible oxygen storage materials for automobile catalysts" and for doping copper oxide superconductors". The diverse cerium enolate precursors and deposition methods used in the formation of cerium oxide thin films are summarized in Table 6, whereby the most common precursor for ceria is Ce(thd)4. 

With the use of the DV-Xa molecular orbital method, electronic structure calculations have been performed to investigate the impurity effect on material properties. Firstly, calculations were done for F atoms substituted for 0 (oxygen) atoms in copper oxide superconductors. It was found that the population of the atomic orbitals of F atoms is small in HOMO (highest occupied molecular orbital) and a small fraction of charge carriers enters the impurity sites. The F impurities are therefore expected to be effective for pinning magnetic flux lines in Cu oxide superconductors. 

The simplest networks are one-dimensional a-networks which may be composed of secondary amides, primary amide dimers or nucleophospholipids. In chapter 5, such structures were discussed as micellar rods and tubules in bulk aqueous solutions. Two-dimensional materials such as copper oxide superconductors, molybdenum sulfide lubricants and intercalated graphites are mostly inorganic. The anisotropic properties are a result of covalent bonds in two dimensions and weak interactions in the third dimension. One may, however, also envision strong hydrogen-bond interactions within an organic layer, whereas adjacent layers are held together only by van de Waals interactions. The two-dimensional, or p-network may form spontaneously from an... 

Unlike other copper oxide superconductors which have higher cja ratio, the cj a ratio in the non-defective infinite-layer compound is close to 1. However, because of a lack of apical oxygen, the electronic structure is expected to be more anisotropic than all other high oxides, despite the similar c and a lattice constants of the unit cell. This supposition was recently confirmed by X-ray absorption spectroscopy (XAS) measurements [8.27], in which an anisotropic upper Hubbard band was reported. In this section, we will discuss the question of whether the electronic transitions near the Fermi surface behave anisotropically with respect to the a6-plane and the c-axis. 

The reason for the chemical vulnerability of phases based on copper oxides is shown in Fig. 1, which is a display of the heat of formation per gram-atom of the most stable solid oxides formed by the metallic and semiconducting elements in the periodic table. To generate this table, compilations of thermochemical data were consulted (23), and these sources provided the enthalpies of formation used for the calculations presented later. The reason for expressing the data in the manner shown in Fig.l is to get an idea of the relative strengths of the M-O bonds that form the solid oxides. One can see immediately that very few elements form weaker bonds with O than does Cu. Thus, most elements will reduce CuO to elemental Cu, and are very likely to react chemically with any of the copper-oxide superconductors as well. Although a positive heat of reaction, determined for interactions of the 1-2-3 superconductors with another material, is not a sufficient condition for chemical stability, it is almost certainly necessary. Consequently, researchers interested in the processing of copper-oxide superconductors into structures should consider thermochemical properties carefully. 

The structural feature common to each of the known copper oxide superconductors is the presence of [Cu02]oo planes. These two-dimensional layers are schematically illustrated in Figure 1. The layers are formally... 

Although the detailed structural chemistry of the molecular and copper oxide superconductors is naturally quite different, there are broader features that they have in common, sufficient to make a comparison worthwhile. 

Furrer, A., ed., 1998, Neutron Scattering in Layered Copper-Oxide Superconductors, Vol. 20 of Physics and Chemistry of Materials with Low-Dimensional Structures (Kluwer Academic Publishers, Dordrecht). 

Fischer, E, and M. Medarde, 1998, in Neutron Scattering in Layered Copper-Oxide Superconductors, ed. A. Furrer, Vol. 20 of Physics and Chemistry of Materials with Low-dimensional Structure (Kluwer Academic Publishers, Dordrecht) pp. 261-301. Fischer, P, B. Schmid, P. Briiesch, F. Stuck and P Untemahrer, 1989, Z. Phys. B 74, 183. 

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