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Item Open Access (BEDT-TTF)2X(TCE)0,5[X:BF4, FSO3, IO4] new organic metals(1991) Armbruster, Kurt; Bele, Petra; Brunner, Hermann; Chen, Benming; Dahm, Stefan; Geiger, Rolf; Heinen, Ilsabe; Keller, Heimo J.; Schweitzer, DieterThe structure, the temperature dependence of the resistivity, thermopower, ESR-linewidth and the spin susceptibility of a new series of organic metals (BEDT-TTF)2X·(TCE)0,5 [X = BF4, FS03 and IO4] is presented and discussed.Item Open Access FT-IR investigations of BEDO-TTF and radical salts of BEDO-TTF(1993) Moldenhauer, Jörg; Pokhodnia, Konstantin I.; Schweitzer, Dieter; Heinen, Ilsabe; Keller, Heimo J.We investigated powder absorption spectra of the donor BEDO-TTF and some radical salts of BEDO-TTF, which are metals even at low temperatures. After an assignment of the bands in the neutral donor we found in the radical salts different vibrational frequencies of bands correlated with C-O vibrations. These frequencies are directly related to the average charge on the donor molecule. Additionally frequencies of CH2-stretching vibrations exhibit differences, which can be ascribed to a varying strength of donor-anion interaction depending on the respective anion.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 Characterisation of the Fermi surface and phase transitions of (BEDO-TTF)2 ReO4·(H2O) by physical property measurements and electronic band structure calculations(1994) Kahlich, Siegfried; Schweitzer, Dieter; Rovira, Concepcio; Paradis, Jeffrey A.; Whangbo, Myung-Hwan; Heinen, Ilsabe; Keller, Heimo J.; Nuber, Bernhard; Bele, Petra; Brunner, Hermann; Shibaeva, Rimma P.The electronic properties of the organic superconductor (BEDO-TTF)2 ReO4·(H2O) were investigated by temperature dependent resistivity, ESR, Hall effect and magnetoresistance measurements. Shubnikov-de Haas (SdH) oscillations were observed in magnetic fields up to 24 T in the temperature range 0.5 K to 4.2 K. The electronic band structure of (BEDO-TTF)2 ReO4·(H2O) was calculated by employing the extended Hückel tight binding method on the basis of its room temperature crystal structure. The two observed SdH frequencies of 75 T and 37 T correspond very well with two cross-sectional areas of the hole and electron Fermi surface pockets obtained from the tight binding calculation. From the temperature dependence of the SdH oscillation amplitudes, the cyclotron effective mass (mc) belonging to the larger and smaller pockets were found to be 0.9 m0 and mc=1.15 m0 respectively. Measurements of the angular dependence of the SdH frequencies show no deviation from that expected for a cylindrical Fermi surface. In terms of our tight binding calculations and experimental measurements, probable causes for the 213 K and ∼35 K phase transitions are discussed. The calculations show that (BEDO-TTF)2 ReO4·(H2O) is a two dimensional semimetal but possesses a hidden nesting. The latter is likely to cause an SDW instability leading to the ∼35 K transition. The resistivity drop associated with the 213 K transition is likely to be induced by an abrupt increase in the relaxation time. The excellent agreement between the calculated and experimentally observed Fermi surface implies that, with decreasing temperature below 35 K, (BEDO-TTF)2 ReO4·(H2O) gradually gets out of the SDW state and re-enters the "original" metallic state, in which it becomes superconducting below 2.4 K.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 FT-IR absorption spectroscopy of BEDT-TTF radical salts: charge transfer and donor-anion interaction(1993) Moldenhauer, Jörg; Horn, Christof; Pokhodnia, Konstantin I.; Schweitzer, Dieter; Heinen, Ilsabe; Keller, Heimo J.IR absorption spectra from various K-phases of BEDT-TTF radical cation salts as well as from α-(BEDT-TTF)2I3 are presented. From tbese spectra one special mode ν27 representing the asymmetric ring C=C stretching vibration is assigned and the dependence of its frequency from the average charge on the donor molecule is given. The investigation of the metal-insulator phase transition of α-(BEDT-TTF)2I3 at 135 K with respect to this vibrational model ν27 gives strong indication of a charge localization in at least one of the two crystallographic different stacks. Furthermore, we correlate the frequencies of the CH2 stretching modes of a variety of BEDT-TTF radical salts with the superconducting phase transition temperatures. These frequencies are evidently not charge dependent, but significantly reflect the strength of the interaction of the donor molecule with the respective anion. It could be shown that phases with a higher Tc of their superconducting transition exhibit a smaller red shift of these frequencies, i.e., a less attractive donor-anion interaction.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 Transport properties of single crystals and polycristalline pressed samples of (BEDT-TTF)2X salts and related coordination polymers(1991) Schweitzer, Dieter; Kahlich, Siegfried; Gärtner, Stephan; Gogu, Emil; Grimm, Hans; Heinen, Ilsabe; Klutz, Thomas; Zamboni, Roberto; Keller, Heimo J.; Renner, GerdTen years ago in 1979, superconductivity was observed for the first time in an organic metal. today, about 30 different organic metals are known, which become superconducting under pressure or ambient pressure. The organic superconductors with the highest transition temperatures are all radical salts of the donor bis(ethylenedithioio)-tettathiafulvalene (BEDT-TTF), namely at ambient pressure (BEDT-TTF)2CU(NCS)2 (Tc = 10.4 K) and αt,-(BEDT-TTF)2I3 (Tc = 8 K) and under isotropic pressure β H- (BEDT-TTF)2I3(0.5 kbar, Tc = 7.5 K) The latter β H-phase can even become superconducting at 8 K and ambient pressure, after a special pressure-temperature cycling procedure i.e. pressurization up to 1 kbar at room temperature, and release of the helium gas pressure at temperarures below 125 K. Nevertheless, this superconducting state at 8 K in β H-(BEDT -TTF)2I3 is only metastable , since warming up the crystal above 125 K and cooling down again under ambient pressure, results only in superconductivity at 1.3 K, the so-called β L - or β-phase.