Browsing by Author "Hennig, Ingolf"

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    α- and β-(BEDT-TTF)2+I3-: two dimensional organic metals
    (1985) Hennig, Ingolf; Bender, Klaus; Schweitzer, Dieter; Dietz, Klaus; Endres, Helmut; Keller, Heimo J.; Gleitz, Arno; Helberg, Hans W.
    Electronic properties of α- and β-(BEDT-TTF)2I3 crystals are reported.
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    (BEDT-TTF)2+J3- : a two-dimensional organic metal
    (1984) Bender, Klaus; Dietz, Klaus; Endres, Helmut; Helberg, Hans W.; Hennig, Ingolf; Keller, Heimo J.; Schäfer, Herbert; Schweitzer, Dieter
    Temperature-dependent thermopower-, dc- and microwave-conductivity measurements on the two-dimensional organic metal (BEDT-TTF)2+J3- are reported.
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    Electric conductivity and thermoelectric power of various polypyrroles
    (1987) Bender, Klaus; Gogu, Emil; Hennig, Ingolf; Schweitzer, Dieter; Münstedt, Helmut
    The dc-conductivity and thermoelectric power of electrochemically prepared polypyrroles have been investigated in the temperature range between 300 K and 1.3 K on unaged, aged (24-120 days at 80°C in air) and chemically modified samples. The linear temperature dependence of the thermopower of unaged polypyrroles demonstrates clearly the contribution of metallic regions to the electronic transport properties. The temperature dependence of the conductivity of the polypyrroles investigated corresponds to that predicted by the variable range hopping model.
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    Electrochemically prepared radical salts of BEDT-TTF: molecular metals and superconductors
    (1987) Schweitzer, Dieter; Gogu, Emil; Hennig, Ingolf; Klutz, Thomas; Keller, Heimo J.
    The structural, electronic and superconducting properties of several electrochemically prepared BEDT-TTF radical salts are discussed.
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    Knight shift tensors and π-spin densities in the organic metals αt-(BEDT-TTF)2I3 and (BEDT-TTF)2Cu(NCS)2
    (1991) Klutz, Thomas; Hennig, Ingolf; Haeberlen, Ulrich; Schweitzer, Dieter
    13C-MASS spectra of pure BEDT-TTF and of the organic metals αt-(BEDT-TTF) 2I 3 and (BEDT-TTF) 2Cu(NCS)2 were recorded at νL - 68 MHz. Isotropic shifts and the principal components of the shift tensors were determined, respectively, from the center and spinning side bands. For pure BEDT-TTF which is a diamagnetic insolator, the measured shifts are chemica/ shifts, while for the organic metals they are the sum of chemical and Knight shifts. In each of the compounds the shifts are assigned in groups to the inner, middle and outer carbons of the BEDT-TTF molecule. For the organic metals the separation of the experimental shifts into chemical and Knight shifts is discussed. From the anisotropic part of the Knight shift tensors the π-spin densities at the carbon and sulphur positions of the BEDT-TTF molecule are inferred. The result is that the π-spin density of the unpaired hole is concentrated on the center part of the BEDT-TTF molecuIe, i.e. on the inner and middle carbons, and on the inner sulphurs. It is argued that the current density is concentrated on this part of the BEDT-TTF molecule as well.
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    NMR investigations of the organic metals and superconductors α-(BEDT-TTF)2I3, β-(BEDT-TTF)2I3 and α t-(BEDT-TTF)2I3
    (1988) Hennig, Ingolf; Haeberlen, Ulrich; Heinen, Ilsabe; Schweitzer, Dieter; Keller, Heimo J.
    1H and 13C spin lattice relaxation rates as well as 13C Knight shifts of the quasi two dimensional organic metals α-, β- and αt-(BEDT-TTF)2I3 are reported. In β- and αt-(BEDT-TTF)2I3 at temperatures below 100 K the Korringa relation is fulfilled. Near the critical temperature of 8K of the stable superconducting state of αt-(BEDT-TTF)2I3 an enhancement of the proton spin lattice relaxation rate due to superconducting fluctuations is observed. The observed individual 13C Knight shifts, as measured by magic angle sample spinning and cross polarisation methods range between -6 and +244 ppm for α- phase and -10 and +125 ppm for β- and αt- phase crystals.
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    A novel molecular metal: (oxamide oximato)(oxamide oxime)nickel(II) tetracyanoquinodimethanide, [Ni(oaoH)(oaoH2)]tcnq, and physical properties of its semiconducting Pt analogue
    (1985) Endres, Helmut; Bongart, August; Nöthe, Dietrich; Hennig, Ingolf; Schweitzer, Dieter; Schäfer, Herbert; Helberg, Hans W.; Flandrois, Serge
    (C4H11N8NiO4)+(C12H4N4)-, Mr = 498.09 is triclinic, p1, a -=3.7718(6), b = 7.436(2), c =17.511(4) A, a=88.67(2), β=86.93(2), γ=85.05(2), γ= 488.51 A 3, Z = 1, d c=1.69 gcm -3, final R w= 0.035 for 1454 observed independent reflections. The crystals consist of segregated regular parallel stacks of planar metal complex cations and tcnq - counterions with intermolecular H bonds stabilizing the structure. The compound is metallic at room temperature. A metal to semiconductor transition around 230 K shows up in thermopower data, in the microwave conductivity and epr around 170 K. It is not visible in the static magnetic susceptibility.
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    Organic metals from simple aromatic hydrocarbons: perylene radical salts
    (1985) Schweitzer, Dieter; Hennig, Ingolf; Bender, Klaus; Endres, Helmut; Keller, Heimo J.
    Crystal structures, temperature dependent thermopower and conductivity measurements of several metallic perylene radical salt phases are discussed.
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    A stable superconducting state at 8K and ambient pressure in αt-(BEDT-TTF)2I3
    (1987) Schweitzer, Dieter; Bele, Petra; Brunner, Hermann; Gogu, Emil; Haeberlen, Ulrich; Hennig, Ingolf; Klutz, Thomas; Swietlik, Roman; Keller, Heimo J.
    We report bulk superconductivity at 8 K and ambient pressure in crystals of α t (BEDT-TTF)2I3. In contrast to the earlier observed metastable superconducting state at 8 K in crystals of β-(BEDT-TTF)2I3 here the superconducting state is stable and the crystals can be prepared by tempering α-(BEDT-TTF)2I3 above 70 °C for several days. ac-susceptibility measurements show that the observed superconducting state at 8 K is a bulk property of the crystals. Resistivity measurements indicate a sharp superconducting transition at 8 K with an onset temperature of about 9 K. The upper critical fields Hc2 at 1.3 K lie between 3 and 11 T depending on the direction of the magnetic field with respect to the crystal axes. ESR- as well as NMR-measurements indicate a total transformation of the α-phase crystals into the new superconducting α t -crystals after tempering.
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    Superconductivity at ambient pressure in BEDT-TTF radical salts
    (1988) Schweitzer, Dieter; Polychroniadis, K.; Klutz, Thomas; Keller, Heimo J.; Hennig, Ingolf; Heinen, Ilsabe; Haeberlen, Ulrich; Gogu, Emil; Gärtner, Stephan
    Crystals of (BEDT-TTF)2Cu(NCS)2 were prepared by several methods and the superconducting transitions investigated by resistivity and ac susceptibility measurements. Depending on the preparation of the crystals a variation of the temperature of the superconducting transition is observed. This variation is manifested in the upper critical fields Hc2 and proton NMR relaxation measurements at temperatures below Tc show it as well. The upper critical fields Hc2 of crystals of αt(BEDT-TTF)2I3 were determined in dependence of the temperature and of the direction of the magnetic field with respect to the various crystal axes by measuring the mid transition of the resistivity and of the rf penetration depth. The data are analyzed with the anisotropic effective mass model in the picture of the Ginsburg Landau (Gl) theory as well as in the picture of a layered superconductor. 13C Knight shifts measured by magic angle sample spinning and NMR cross polarisation methods support the picture of the layered superconductor.
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    Synthesis, structure and physical properties of a two-dimensional organic metal, di[bis(ethylenedithiolo)tetrathiofulvalene] triiodide, (BEDT-TTF)2+I3-
    (1984) Bender, Klaus; Hennig, Ingolf; Schweitzer, Dieter; Dietz, Klaus; Endres, Helmut; Keller, Heimo J.
    Electrical oxidation of bis(ethylenedithiolo)tetrathiofulvalene (BEDT-TTF) 1 in a nitrogen saturated tetrahydrofurane solution containing (n-C4H10)4N+I3 as electrolyte results in the deposition of crystal plates of (BEDT-TTF)+2I3 (C10H8S8)2I3. These two dimensional crystals are triclinic, space group P1, a = 9.211(2), b=10.850(4), c = 17.488(5) Aring, agr=96.95(2), β = 97.97(2), γ = 90.75(2)°, V = 1717 Aring3, Mr = 1150.01, Z=2, Dx =2.22 g cm-3. Rw = 0.037 for 2462 observed reflections. Sheets of alternating cations and anions occur in the compound. The most prominent intermolecular S-S contacts (∼3.5Aring) are found between the stacks of BEDT-TTF. The stacking of the molecules and the interstack contacts give the compound a two-dimensional character which influences the physical properties.
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    TCNQ salts of planar metal complex cations: novel molecular conductors and semiconductors
    (1985) Endres, Helmut; Bongart, August; Nöthe, Dietrich; Hennig, Ingolf; Schweitzer, Dieter; Helberg, Hans W.; Schäfer, Herbert
    The facile variation of positive charge of oxamide oxime metal complexes, caused by acid-base equilibrium, allows the growth of single crystals of their TCNQ salts. 1:1 salts consist of reqular segregated stacks of the components, with metallic room temperature behaviour of the Ni compound, the Pt compound being a semiconductor. Room temperature conductivities are of the order of 10 Siemens per cm. A 2:3 Pt complex TCNQ salt contains segregated acceptor stacks with half a negative charge per molecule. These stacks run perpendicular to mixed stacks -D-D-A-D-D-A-, with integral charges on donors D and acceptors A.