The oxygen contents in YBa2CuaOz samples have been determined by two wet chemical redox methods, i.e. by Coulometric and iodometric tirations. Three different samples, synthesized by solgel, UF catalyst and 0, hot-isostatic-press (HIP) techniques, are studied. The results of the two titration methods agree well with each other for all samples. The oxygen content z appears to be dependent on the oxygen pressure applied during the synthesis, since z values slightly below the stoichiometric number of 8.00 have been obtained for the samples prepared under 1 atm oxygen pressure, while for the O2 HIP treated sample the ObSeNed oxygen content is z = 8.00 f 0.01.

A series of neutral dinuclear homo-and heterochiral complexes formed by hydrazine (a molecule with axial chirality), a metal atom of group 10 (Ni, Pd, Pt), and halogen atoms (F, Cl, Br, I) was studied by computational methods including density functional theory. In the optimized structures the heterochiral complexes were always found to be energetically favored. The chiral discrimination depends on the contributions coming from the metal atoms and the halogen anions attached to them. Moreover, the relative energies of the optimized complexes are correlated with the electronegativity of the metals and the halogens as well as with the relative distances of the bond critical points in the metal-nitrogen bonds. Also the smaller deviation of the ligand-metal-ligand angles from the ideal value of 90°corresponds to the heterochiral complexes. An analysis of the bonding properties reveals a complex mechanism for the behavior of the electron lone pairs of the nitrogen and halogen atoms. The electronic

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Bulk YBa2(Cul-y57Fey)40s samples with y = O.OM.05 were synthesized by an aqueous sol-gel method under 1 atm oxygen pressure. The samples were characterized by XRD for phase purity and lattice parameters, by thermal analysis for oxygen stability as well as by magnetic susceptibility and SQUID measurements for magnetic and superconducting properties. Iron was found to both destabilize the YBa2Cu408 phase and have a strong effect on its properties even at very low concentrations. The site assignment proposed for the iron dopant is based on the results of 57Fe Mossbauer spectroscopy studies. The results are compared with the observed iron doping effects in the YBa2Cu307-s phase.

Several possible flux materials have been investigated for preparing the superconducting YBa, Cu, O, phase under 1 atm of oxygen. LiF was a promising flux and the YBa, Cu, O, phase was successfully obtained at 815 "C with 0.5-1 .O wt.% LiF. Large amounts of LiF (> 1 .O wt.%) accelerated the formation of side products, i.e. BaCuO, and Y,BaCuO,, instead of the superconducting phase. LiF was also used as a flux in the syntheses of the superconducting Yo,,Cao., Ba, Cu, O, and EuBa,Cu,O, phases, and synthesis conditions for these compounds were optimized.