Browsing by Author "Gärtner, Stephan"
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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 BEDT-TTF-Salze mit quadratischen Platinaten(II) als Gegenionen: [BEDT-TTF]4[Pt(CN)4](1990) Gärtner, Stephan; Heinen, Ilsabe; Keller, Heimo J.; Niebl, Roland; Nuber, Bernhard; Schweitzer, DieterIm folgenden beschreiben wir Synthese, Kristallstruktur und das physikalische Verhalten verschiedener Modifikationen einer weiteren Verbindung aus dieser Reihe, dem [BEDT-TTF]4[Pt(CN)4]. Über eine bei Raumtemperatur metallische Modifikation dieses Festkörpers wurde kürzlich berichtet. Diese Modifikation wird im folgenden als [BEDT-TTF]4[Pt(CN)4'] bezeichnet und hat die folgenden Zelldaten: triklin,α=11,002(9) A,b=17.906(9)A,c=16,625(7)A, a=77.28(4)°,β=84,17(5)°,γ=81.26(6)°. Raumgruppe CI,Z=2[5]. Uns gelang nun die Isolierung und die physikalische Charakterisierung von drei weiteren Festkörpern mit der genannten Zusammensetzung. Wir nennen diese β-,γ- und δ-Phase.Item Open Access Bulk superconductivity at ambient pressure in polycrystalline pressed samples of organic metals(1990) Schweitzer, Dieter; Kahlich, Siegfried; Gärtner, Stephan; Gogu, Emil; Grimm, Hans; Zamboni, Roberto; Keller, Heimo J.Bulk superconductivity in polycrystalline pressed samples of αt-(BEDT-TTF)2I3 and βp-(BEDT-TTF)2I3 is reported. This finding shows that organic superconductors can be used in principle for the preparation of electronic devices and superconducting cables.Item Open Access Bulk superconductivity in polycrystalline pressed samples of αt-(BEDT-TTF)2I3 and βp-(BEDT-TTF)2I3(1990) Schweitzer, Dieter; Kahlich, Siegfried; Gärtner, Stephan; Gogu, Emil; Grimm, Hans; Zamboni, Roberto; Keller, Heimo J.Bulk superconductivity at ambient pressure in pressed samples of polycrystalline αt-(BEDT-TTF)2I3 and βp-(BEDT-TTF)2I3 is reported. This finding is remarkable because it demonstrates that organic superconductors can be used in principal for applications such as the preparation of electronic devices and superconducting cables. In addition the observation of bulk superconductivity in large pressed samples of crystallites of a typical diameter of 1μm of organic metals indicates that superconductivity in organic polymers should be observable as well.Item Open Access CH2 vibrations in radical cation salts based on the donor BEDT-TTF(1991) Gärtner, Stephan; Schweitzer, Dieter; Keller, Heimo J.Investigations of the CH2 stretch vibrations in radical cation salts based on the donor BEDT-TTF and the neutral donor itself are presented. The spectra taken by Fourier transform infrared absorption technique not only demonstrate the importance of the relationship between the possible positions of the CH2 groups in the BEDT-TIF molecule and the physical properties of the organic metal but also offer the opportunity to achieve structural information by the use of a simple method.Item Open Access Detection of CH2 stretch vibrations in α-, αt- and β-(BEDT-TTF)2I3 by FT-IR absorption spectroscopy(1991) Gärtner, Stephan; Schweitzer, Dieter; Keller, Heimo J.We have investigated the CH2 stretch vibrations of α-, αt- and β-(BEDT-TTF)2I3 as well as those of the protonated and deuterated pure donor BEDT-TTF by FT-IR absorption spectroscopy. The crystals were pulverized and diluted in pressed pellets of KBr. Results in the spectral range around 2900 cm-1 taken at room temperature are presented. The spectra clearly reveal absorption lines varying in position and intensity between the different species in a characteristic manner.Item Open Access Organic metals from chiral BEDT-TTF donors(1991) Chen, Ben-ming; Deilacher, Frank; Hoch, Matthias; Keller, Heimo J.; Wu, Pei-ji; Gärtner, Stephan; Kahlich, Siegfried; Schweitzer, DieterWe have shown that is is possible to obtain organic metals from chiral molecules. The X-ray results prove an oxidation number of + 1.5 for the radical cations in at least two cases. To our best knowledge. these are: the first examples of BEDT-TTF-related radical cation salts with this oxidation number. As expected, we obtained statistically disordered crystals, containing the two different enantiomers in a "racemic" mixture. The broad smeared-out phase transitions are probably due to this disorder, or may be caused by the three-dimensional interactions which have been observed, so far, only once in a BEDT-TTF radical salt. We will crystallize the metallic compounds using "optically pure" isomers, in the hope to be able to isolate chiral metals.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 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 Static and ESR susceptibilities of the ambient pressure superconductors β- and αt-(BEDT-TTF)2I3 and (BEDT-TTF)2Cu(SCN)2(1988) Klotz, Stefan; Schilling, James S.; Gärtner, Stephan; Schweitzer, DieterWhereas the static spin-susceptibility χ(T) of β- and αt-(BEDT-TTF)2I3 at 5.7 tesla increases in a slow monotonic manner with temperature, χs(T) for the organic superconductor (BEDT-TTF)2Cu(SCN)2 displays a markedly structured temperature dependence. This gives evidence for two or more phase transitions in the metallic phase at temperatures near 100 K and 50 K, in addition to the onset of superconductivity at 5.7 tesla below not, vert, similar 8 K. Good agreement is found between the absolute magnitudes of static and ESR susceptibility results, thus confirming that orbital susceptibility contributions are of negligible importance here. Cooling in low magnetic fields reveals that superconductivity in (BEDT-TTF)2Cu(SCN)2 sets in at a temperature near 8.5 K, with a 7.6% Meissner effect in 14 G field.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 Superconductivity in polycrystalline pressed samples of organic metals(1989) Schweitzer, Dieter; Gärtner, Stephan; Grimm, Hans; Gogu, Emil; Keller, Heimo J.We report bulk superconductivity at ambient pressure in polycrystalline pressed samples of αt-(BEDT-TTF)2I3. As in single crystals of αt-(BEDT-TTF)2I3 the onset for superconductivity in the polycrystalline pressed samples lies at about 9K but the transition is broader. Zero resistivity is observed at 2.2K and the middle of the transition is found at 5K. The measurement of the ac susceptibility shows that at 2K about 50% of the sample is superconducting, but at this temperature the transition is still not saturated. The observation of bulk superconductivity in this polycrystalline pressed samples is of special interest because it demonstrates that organic superconductors can be used in principle as well for the production of electronic devices such as squids and superconducting cables. In addition the finding of bulk superconductivity in large pressed samples of crystallites of the typical diameter of 1μm of an organic metal indicates that the observation of superconductivity in organic polymers should be possible as well.Item Open Access Synthesis, structure and electrical properties of the two-dimensional organic conductor, (BEDT-TTF)2BrI2(1986) Zhu, Daoben; Wang, Ping; Wan, Meixiang; Yu, Zhaolou; Zhu, Nailue; Gärtner, Stephan; Schweitzer, DieterSingle crystals of α-(BEDT-TTF)2BrI2 and β-(BEDT-TTF)2BrI2 were prepared using standard electrochemical techniques in nitrogen saturated benzonitrile using containing (n-C4H10)NBrI2 as supporting electrolyte. The crystals have nearly identical structure features of α-(BEDT-TTF)2I3 and β-(BEDT-TTF)2I3, except that the BrI−2 anions are disordered in α-(BEDT-TTF)2BrI2 and β-(BEDT-TTF)2BrI2. Their electrical behavior is different from the corresponding α-,β-(BEDT-TTF)2I3 species.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.