E. Kemnitz

Electrochemical evidence for Cu3+-Cu2+-Cu+ transitions in the orthorhombic YBa2Cu3O7-x phase

Comparative studies of the electrochemical behaviour of solid YBa2Cu3O7-x, CuO and Cu2O were performed, using 1M NaOH as electrolyte and applying the method of abrasive stripping voltammetry. The results show that high-temperature superconductivity is connected (i) with the observation of two one-electron steps between Cu3+, Cu2+ and Cu+ at +0.658 V and +0.052 V versus SRE and (ii) with an inability of Cu+ to be reduced to Cu0. The occurrence of the two one-electron steps Cu3++e- to Cu2+ and Cu2++e- to Cu+ supports the disproportionation theory of high-temperature superconductivity, which assumes the formation of Cooper pairs of electrons due to disproportionation of Cu2+. A decrease of the oxygen content of YBa2Cu3O7-x from x=0 to x=0.7 is accompanied by the disappearance of the one-electron reduction step Cu2++e- to Cu+.

XPS analysis of β-AlF3 phases with Al successively substituted by Mg to be used for heterogeneously catalyzed Cl/F exchange reactions

A new method of static charge referencing was applied in a XPS study of β-AlF3 phases with aluminum successively substituted by magnesium. This class of compounds is characterized by strong Lewis acid sites, which are assumed to be the catalytically active sites for Cl/F exchange reactions. In order to adjust their strengths Mg was introduced into the β-AlF3 lattice. At low amounts of Mg the resulting samples were found to be highly catalytically active in Cl/F exchange reactions. 20 nm gold particles were deposited on the sample surface to provide a static charge reference. With the help of this procedure binding energies (BE) of photo peaks as well as kinetic energies (KE) of Auger electron emission peaks are presented in relation to the Au4f7/2 BE reference. Furthermore, a detailed analysis of the relaxation behavior of the Mg nucleus lead to the conclusion that, at low Mg contents, strong Lewis acidic Mg cations in MgxFy clusters act as the catalytically active sites. At high Mg contents, small MgF2 like crystallites are formed in the disturbed β-AlF3 lattice, which are catalytically inactive as MgF2.

Determination of the complete stoichiometry of Bi-Pb-Sr-Ca-Cu-O-superconductors

SummaryA convenient analytical method based on complexometric and iodometric titration for determining the complete stoichiometry of Bi-Pb-Sr-Ca-Cu-O-superconductors (PBSCCO) has been developed. It is shown that the individual contents of Cu3+, Bi5+, Pb4+ are not required for calculating the oxygen content of such materials. It turned out that the lead content and the oxygen content of sintered samples are much lower than those of calcined powders.

A simple and convenient solid state microanalytical technique for identification and characterization of the high temperature superconductor YBa2Cu3O7−x

SummaryThe application of a new electroanalytical technique, abrasive stripping voltammetry, is described for the purpose of identification and characterization of orthorhombic YBa2Cu3O7−x. The method is based on the peculiar property of this compound that the copper ions therein cannot be reduced below the +1 oxidation state, whereas all other copper compounds exhibit this property. The total analysis takes about 2 min and is very reliable.

Non-oxidative and oxidative chlorination of (Bi,Pb)2Sr2Ca2Cu3Oy

The influence of oxidative and non-oxidative chlorination on the chemical and superconducting properties of (Bi,Pb)2Sr2Ca2Cu3Oy is studied on samples with different initial O content. Owing to oxidative chlorination of O-deficient phases a significant increase of Tc is observed although insulating, hygroscopic halide phases are formed and indications for the incorporation of Cl into the structure are not found. Obviously, reactive O is formed during degradation of the 2223 phase on the solid-gas phase boundary. The released O partially occupies anion vacancies in the unreacted O-deficient material resulting in improved intragranular superconducting behaviour. The reaction pathways of the heterogeneous solid-gas reaction are discussed on the basis of the results of chemical analysis.