15 Fakultätsübergreifend / Sonstige Einrichtung
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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 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 (BEDO-TTF)2ReO4·(H2O): a new organic superconductor(1991) Kahlich, Siegfried; Schweitzer, Dieter; Heinen, Ilsabe; Song, En Lan; Nuber, Bernhard; Keller, Heimo J.; Winzer, Klaus; Helberg, Hans W.The structure and the temperature dependence of the resistivity, thermopower and ac-susceptibility of the new organic metal (BEDO-TTF)2ReO4(H2O) was investigated. The resistivity and thermopower data indicate phase transitions at 213K, around 90K and 35K. Below 2.5K an onset to superconductivity is observed in the resistivity data. Superconductivity was suppressed in the resistivity at 1.3K by applying a magnetic field of about 0.2T. Ac-susceptibility data indicate that superconductivity is a bulk effect in (BEDO-TTF)2ReO4(H2O) but the onset for superconductivity observed in the ac-susceptibility is only at 0.9K and the transition seems to be complete only at temperatures below 50 mK. This broad transition might be due to some disorder in the structure created by the low temperature phase transitions.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.