Gerhard Gritzner

Solvent effects on redox potentials: Studies in N-methylformamide

Abstract The redox behaviour of LiClO4, NaClO4, KClO4, RbClO4, CsClO4, TiClO4, Ba(ClO4)2, Cu(CF3SO3)2, Cd(CF3SO3)2, Zn(CF3SO)2, Zn(ClO4)2·2 H2O, (Et4N)3 Fe(CN)6, (Bu4N)3Fe(CN)6, (Et4N)3 Mn(CN)6, (Bu4N)3 Mn(CN)6, ferrocene (biscyclopentadienyl-iron(II), bis(biphenyl)chromium tetraphenylborate and perylene has been studied in N-methylformamide employing polarography and cyclic voltammetry. Standard redox potentials have been estimated from polarographic half wave potentials—measured versus bis(biphenyl)chromium as a reference redox system—of reversible or nearly reversible electrode processes. The data have been compared with the redox potentials versus bis(biphenyl)chromium in formamide and N,N-dimethylformamide. Since the dielectric constants of these three solvents differ considerably, models for the correlation of redox potentials with solvent parameters have been evaluated with regard to their ability to predict differences in redox potentials of a given system in these solvents. Purely electrostatic concepts could not account for the observed changes in redox potentials from one solvent to another. Considering interactions of the solvent molecules as donors with cations as acceptors yielded a good correlation between the donor number of the solvents and the redox potentials for cations that were reduced to the respective metal amalgams. The redox potentials of the hexacyanomanganate and hexacyanoferrate anions were found to be a function of the acceptor properties of the solvents.

Effects of Sn, Co and Fe on MgB2

Abstract The addition of Sn to the preparation of MgB 2 led to the formation of Mg 2 Sn. Both Co and Fe were to some extent incorporated into the MgB 2 lattice. Small amounts of Co or Fe decreased the T c(0) -values but did not destroy the superconducting behavior of MgB 2 . Larger amounts of Co produced amorphous material and CoB. Fe 2 B was found at higher Fe contents as a secondary phase.

Single-ion transfer properties: a measure of ion-solvation in solvents and solvent mixtures

Single-ion transfer properties such as Gibbs energies, entropies and enthalpies may be successfully employed to account for the interactions of ions with solvents. Several electrochemical methods such as polarography or potentiometry in combination with extrathermodynamic assumptions, e.g. the assumption of a reference redox system, the assumption of a negligible liquid junction potential or the assumption of a negligible thermal diffusion potential permit the estimation of single-ion transfer properties. These data are a basis for the understanding of the interactions of ions with solvents and solvent mixtures. Gibbs energies of transfer may be used to learn about the nature of the chemical bond between ions and solvent molecules and to quantize the donor and the acceptor properties of solvents. Entropies of transfer (TΔtS0-values) yield information of the effects of the solvated ions on the surrounding solvent matrix. The contributions from the Gibbs energies and the TΔtS0-values allow to perceive the enthalpies of transfer. Single-ion transfer properties into solvent mixtures show the effects of preferential solvation.

Gibbs energies, enthalpies and entropies of transfer of cations from acetonitrile and N,N-dimethylformamide into water

Gibbs energies of transfer from acetonitrile and N,N-dimethylformamide into water for the cations Li+, Na+, K+, Rb+, Cs+, Ag–, Tl+, Ba2+, Cu2+, Zn2+, Cd2+, and Pb2+ have been calculated from reversible polarographic half-wave potentials on the basis of the bis(biphenyl)chromium assumption. Gibbs energies of transfer for Ag+ and Hg2+ were obtained from the potentials of the respective M/Mn+(0.01 mol dm–3) electrodes. Good agreement was found with Gibbs energies of transfer obtained from the tetraphenylarsonium tetraphenylborate assumption. Reaction entropies for the systems Mn+/M(Hg) and entropies of transfer have been derived from measurements of the temperature coefficents of the polarographic half-wave potentials vs. Ag/Ag+(0.01 mol dm–3) electrodes kept at 25 °C employing the assumption of a negligible thermal diffusion potential. Enthalpies of transfer were obtained from the Gibbs energies and the entropies of transfer.

Fabrication of ceramic Y1−xCaxBa2Cu4O8+δ and YBa2Cu3O7 superconductors from oxalate precursors at ambient pressure

Abstract Ceramic Y 1− x Ca x Ba 2 Cu 4 O 8+δ and YBa 2 Cu 3 O 7 superconductors were prepared from coprecipitated oxalate precursor material. Calcination and firing at 820°C under atmospheric oxygen pressure yielded pure phase Ca 2+ doped and undoped YBa 2 Cu 4 O 8+δ superconducting material. The T co values increased from 80 to 85 K with an increase in the Ca 2+ content from x =0 to 0.1. YBa 2 Cu 3 O 7 was obtained after calcination of the oxalates at 860°C and sintering at 920°C under ambient oxygen pressure. This superconductor had a T co of 92 K with a transition width of 1 K.

Solvent effects on electrode potentials of metalloporphyrins: Reduction of 5,10,15,20-tetraphenylporphinato complexes in non-aqueous media

Abstract Reduction potentials were collected for 16 different tetraphenylporphyrin complexes which form anion radicals and dianions in non-aqueous media. Data were collected in up to 13 solvents in order to formulate a model of how solvent-metalloporphyrin interactions affect half-wave potentials for stepwise reduction of the different metalloporphyrins. Analysis of the data was by both linear regression and principal vector analysis. The solvent parameters investigated were the Gutmann donor number (DN), the Gutmann acceptor number (ACN), the Dimroth-Reichardt parameter (ET), the solvent dielectric constant (e), and the Taft parameters (α,β,π*). In almost all cases good correlations were found with the solvent acceptor properties ACN and ET. The implications of this correlation in terms of modulating redox potentials is discussed.

Properties, magnetic susceptibility, critical currents and irreversibility fields of (Tl0.5Pb0.5)Sr2 (Ca1−xGdx)Cu2Oz bulk superconductors

Bulk (Tl0.5Pb0.5)Sr2(Ca1−xGdx)Cu2Oz superconductors with x = 0.1,0.2 and 0.3 were prepared by a wet chemical gel technique followed by a special heat-treatment procedure. X-ray diffraction analysis revealed practically phase-pure samples with tetragonal structure (space group P/4mmm). The temperature dependence of the real and imaginary parts of the ac susceptibility and the dependence on the ac applied magnetic field were investigated to learn about the superconducting and magnetic properties of bulk (Tl0.5Pb0.5)Sr2(Ca1−xGdx)Cu2Oz superconductors. The critical current densities of the samples were derived from Beans model. The temperature dependence of the critical current was successfully fitted based on a power law from the thermally activated magnetic flux creep model. From magneto-resistance measurements the temperature dependences of the transport critical currents and the irreversibility fields were found. The dc magnetic field dependence of the width of superconducting transitions and the critical currents was also obtained. These properties of the superconductors were analyzed within available theoretical models.

Temperature coefficients of half-wave potentials and entropies of transfer of cations in aprotic solvents

Temperature coefficients of polarographic half-wave potentials for Li+, Na+, K+, Rb+, Cs+, Tl+, Cu2+, Zn2+, Cd2+, Ba2+, Pb2+, bis(biphenyl)chromium(I) tetraphenylborate, ferrocene and perylene have been derived from measurements vs. Ag/Ag+(0.01 mol dm–3) electrodes kept at 25 °C in 0.1 mol dm–3 solutions of tetrabutylammonium perchlorate in dimethyl sulphoxide, N,N-dimethylformamide, hexamethylphosphoric triamide and propylene carbonate. Entropies for the above mentioned redox couples and for Ag+ were calculated for reversible electrode reactions on the basis of the assumption of a negligible thermal diffusion potential. This assumption was also employed to calculate single-ion entropies of transfer from N,N-dimethylformamide into dimethyl sulphoxide, propylene carbonate and hexamethylphosphoric triamide.

Local environments of iron and cobalt in doped MgB2 superconductors

MgB2 has been doped with 57Fe and 57Co in order to probe the electronic structure of the superconductor by Mossbauer spectroscopy. Simultaneous SEM–EDX studies showed that both Fe and Co were incorporated to some extent into the MgB2 host lattice. Mossbauer spectroscopy indicated that Fe2B and FeB were also formed during the preparation. At higher Fe contents Fe2B appeared unambiguously as a secondary phase in the x-ray diffraction. CoB was detected at substitution levels of 10 mol% and greater. Both Fe and Co doping decreased the Tc(0) modestly. Co and Fe were found to substitute at the Mg site in the MgB2 lattice. The Mossbauer parameters for 57Co and 57Fe were consistent with a metallic environment. The similarity of the isomer shifts in MgB2 and in cuprate superconductors is being discussed.

Polycrystalline (Tl0.5Pb0.5)(Sr0.9Ba0.1)2Ca2Cu3Oy superconducting thick films with high critical current densities

Superconducting (Tl0.5Pb0.5)(Sr0.9Ba0.1)2Ca2Cu3Oy films were fabricated on lanthanum aluminate substrates by screen printing of the precursor material followed by ex situ thallination. The thickness of the films varied between 1.3 and 2.6 μm. X-ray diffraction and optical microscopy showed that the films were highly textured. Samples with critical temperatures Tc(0) between 116 and 120 K and with transition widths ΔT(90−10%) of 1 K have been fabricated. Critical current densities between 0.6 and 0.9 MA cm−2 were measured.