Universität Stuttgart
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Item Open Access Radical cation salts of BEDT-TTF and an unsymmetrical BEDT-TTF derivative(1986) Schweitzer, Dieter; Bender, Klaus; Endres, Helmut; Gärtner, Stephan; Heid, Rolf; Heinen, Ilsabe; Keller, Heimo J.; Weber, Angelika; Weger, MeirInvestigations of several BEDT-TTF and DIMET radical salts are presented. Measurements on β-(BEDT-TTF)2IAuI crystals show that their conducting and superconducting properties might vary strongly depending on the decomposition of (IAuI)− anions during the electrochemical process.Item Open Access Radical cation salts of an unsymmetrical BEDT-TTF derivative: DIMET(1987) Bender, Klaus; Endres, Helmut; Gärtner, Stephan; Gogu, Emil; Heid, Rolf; Heinen, Ilsabe; Keller, Heimo J.; Kraatz, Andrea; Schweitzer, DieterA surprising fact in the structures of the organic superconductors (TMTSF)2 Cl04 and -β-(BEDT-TTF)2X (X=I3-, IBr2- and AuI2-) is that there exists a slight dimerisation of the donor molecules within the stacks of these radical cation salts. Therefore, the question arises whether a more pronounced dimerisation in organic radical salts of this type leads to stronger intra- and interstack interactions and to stronger couplings between the unpaired electrons. This might result in an increase of the transition temperature to superconductivity. A step in this direction is the preparation of "unsymmetrical" donors like DIMET, which was synthesized recently. This new donor was electrocrystallized by us in different solvents using tetrabutylammonium salts of various anions like ClO4-, ReO4-, NO3-, I3-, (AuI2)-, PF6-, ASF6- and SbF6- as electrolytes. With all these anions radical salts were obtained and in some cases even several crystallographic different phases.Item Open Access BEDT-TTF salts with square platinates(II) as counterions : [BEDT-TTF]4[Pt(C2O4)2], a new organic metal(1989) Gärtner, Stephan; Heinen, Ilsabe; Schweitzer, Dieter; Nuber, Bernhard; Keller, Heimo J.The title compound, [BEDT-TTF]4[Pt(C2O4)2] is obtained by electrocrystallization, which yields black platelets of stoichiometry C44H32O8PtS32, Mr = 1909.87. Crystal structure determination at room temperature gives a triclinic cell Pi, with a = 8.678(2) A, b =11.878(5) A, c = 15.757(7) A,α = 105.49(3)°, β= 91.05(3)", 'γ= 91.96(3)", V = 1563.64 A3,d c = 2.03 g/cm 3, Z = 1. The solid contains sheets of BEDT-TTF cations separated by sheets of [Pt(C2O4)2]2-- counterions. The BEDT-TTF sheets are made up of columns that interact strongly through S-S contacts. The columns are built up by pairs of BEDT-TTF molecules. There are strongly varying bond distances in the two different BEDT-TTF moieties that make up a pair. This fact is especially clearly expressed in the surprisingly short C-C distances of the central C=C double bond of the two BEDT-TTF ions:1.273 A and 1.327 A respectively. These are by far the shortest distances for the central C=C bonds observed so far in conductive BEDT-TTF salts. The compound behaves like a metal down to about 60 K. Temperature-dependent d.c. conductivity, e.s.r. and thermopower measurements show metallic behaviour at room temperature (σ300 = 20 S/cm) with a metal-to-metal phase transition around 200 K. At about 60 K a second broad phase transition occurs and the crystals become semiconducting.Item Open Access Superconductivity at 10 K and ambient pressure in the organic metal (BEDT-TTF)2Cu(SCN)2(1988) Gärtner, Stephan; Gogu, Emil; Heinen, Ilsabe; Keller, Heimo J.; Klutz, Thomas; Schweitzer, DieterWe confirm the observation of superconductivity at ambient pressure above 10 K in the organic metal (BEDT-TTF)2Cu(SCN)2 as reported recently by Urayama et al [12]. In addition we have measured ESR, ac-susceptibility and thermopower in crystals of (BEDT-TTF)2Cu(SCN)2 and have shown that in contrast to other organic superconductors here a relatively sharp superconducting transition even in the ac-susceptibility can be observed which saturates already around 8 K. The thermopower measurements indicate a clear metal-metal phase transition at 100 K and a possible second phase transition at around 50 K, while from the temperature dependence of the resistivity and susceptibility (ESR) these phase transitions cannot be observed.Item Open Access 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, StephanCrystals 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.Item Open Access 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.Item Open Access Preparation, structure and investigations of BEDT-TTF trihalides(1985) Endres, Helmut; Hiller, M.; Keller, Heimo J.; Bender, Klaus; Gogu, Emil; Heinen, Ilsabe; Schweitzer, DieterCrystals of the β-phases of (BEDT-TTF)2Br-I-Br (1) and (BEDT-TTF)2I-I-Br (2) have been isolated by electrocrystallization. The solids can be obtained using different tetrabutylammonium trihalides containing iodine and bromine as supponing electrolytes. Cyclovoltammetric results show clearly that the trihalide anions are involved in the oxidation reactions near the anode which finally lead to the crystals.Item Open Access Preparation, structure and physical properties of BEDT-TTF nitrates(1985) Weber, Angelika; Endres, Helmut; Keller, Heimo J.; Gogu, Emil; Heinen, Ilsabe; Bender, Klaus; Schweitzer, DieterElectrocrystallization of BEDT-TIF in THF with tetrabutylammonium nitrate as supporting electrolyte leads to the deposition of at least three different phases. Sheets of BEDT-TTF radical cations with short intermolecular S···S contacts (>3.315(4) A) are separated by anion layers. Two more BEDT-TrF nitrates have been characterized by their unit cell data. Resuhs of temperature dependent electrical conductivity and thermopower measurements on crystals of the a-phase are presented. They are metallic down to about 30 K.Item Open Access The double-stack structure of di(3,4-ethylenedithio-3',4'-dimethyl-2,2',5,5'-tetrathiafulvalenium)perchlorate, (DIMET)2ClO4(1987) Endres, Helmut; Heid, Rolf; Keller, Heimo J.; Heinen, Ilsabe; Schweitzer, Dieter2C10H10S6 1/2+.ClO-4 , M r= 744·62, triclinic, PI, α=7·000 (2), b =7·824 (3), c = 27·010 (14) A, α = 88· 10 (4), β= 89·02 (4), Y = 74· 58 (3)°, V = 1425 A 3, Z=2, Dx=1·73gcm-3, λ( MoKa) = 0·7107 A, μ= 10·1 cm-1, F(000) = 762, room temperature, final R = 0.063 for 3076 observed independent reflections. Two crystallographically independent DIMET molecules each with an average charge of ½+ form stacks, which run nearly perpendicularly to one another. Within each stack, two different interplanar spacings between adjacent parallel molecules are found. These overlap patterns are similar to those found in tetrahydrofuran solvate. The stacks are arranged side by side perpendicular to the long molecular axis to give a sheet-like arrangement.Item Open Access Radical cation salts of an unsymmetrical BEDT-TTF derivative: molecular structure and physical properties of (DIMET)2ClO4xTHF(1985) Heid, Rolf; Endres, Helmut; Keller, Heimo J.; Gogu, Emil; Heinen, Ilsabe; Bender, Klaus; Schweitzer, DieterSheets of cations with an average charge of 1/2+ are separated by sheets of anions and THF solvent molecules. Intermolecular S···S contacts result in two-dimensional physical properties. Electrical conductivity and thermopower measurements indicate two broad phase transitions at around 200 K and 80 K.