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Influence of Local Structure on Superconducting Properties of Samples in YBaCuO and Related Systems

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The scientific goal of the Working Group is the determination of the local structure of Cu oxide high Tc superconductors. Samples of Cu oxide superconductors contain superstructures, microwins, non stoichiometry in the anion sublattice, cation ordering, point and extended defects, and intergrowth. With such samples, if we limit ourselves to Bragg scattering, we can only determine the average structure. In order to approximate the local arrangement we are: (i) using more than one radiation (X-ray, neutrons, electrons); (ii) taking into account the diffuse scattering; (iii) carrying out high resolution electron microscopy; (iv) using absorption techniques (EXAFS and XANES).
Research has been carried out in order to determine the local structure of samples of high critical temperature superconductors.
Some of the most important contributions made are:
the synthesis of new copper oxide superconductors obtained by replacing, in known copper-oxide compounds, copper polyhedra by anionic groupings, such as carbonate, sulphate, nitrate and borate (for example, (Y1.6CA0.4)2Ba4Cu5(CO3,NO3)O11 and TISr4-xBaxCu2CO3O7 (x less than 2 and greater than 1) become superconductive at 82 and 62 K respectively);
the synthesis of a new mercury bearing copper oxide compound, HgBa2CuO4+d, which was found to become superconducting at 94 K (this proved to be the first member of a new homologous series of high critical temperature of superconductors, HgBa2Cal-nCunO2n+2+d, where a record critical temperature of 134 K was found for the third member);
the study of the microstructure of YBa2(Cul-xC0x)307-d and its correlation with the superconductivity properties (the twin planes in superconductivity yttrium barium copper oxide (YBCO) samples act as pathways for magnetic flux creep rather than as pinning centres and intergrain flux percolates along the twin walls into the superconducting grains and intergrain critical current density is reduced by intergrain mesostructural properties);
the study of the phase diagram of YBa2Cu306+X for x greater than 0.85 but less than 1.0 (there is strong evidence that the oxygen stoichiometry of the compound corresponding to the highest critical temperature is about 6.9).
POTENTIAL

The new oxide high Tc superconductors obtained by replacing Cu polyhedra by carbonate, sulphate, nitrite and boronate groupings open up the possibility of changing the distances between the CuO2 superconducting layers on an ad hoc basis. The replaced Cu polyhedra are those which do not participate directly in the superconducting mechanisms.

The new Hg-bearings series, HgBa2Ca(1-n)CunO(2n+2+d) , is also very promising because its members exhibit the highest Tc's. For example, for n = 1, 2, and 3, Tc (onset) is 94, 126, and 134 K, respectively.

The technique of relating the twin structure of 123 samples with the critical current density, developed by the Darmstadt group, also has good potential.

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