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Three 2 : 1 salts of the organic donor molecule bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET) with trifluoromethylsulfonyl-based anions N(SO2CF3)2−, CH(SO2CF3)2− and C(SO2CF3)3− were prepared by electrocrystallization. These salts were characterized by single-crystal X-ray diffraction, electron spin resonance (ESR) spectroscopy, electrical resistivity measurements and electronic band structure calculations. (ET)2N(SO2CF3)2 is a two-dimensional (2D) metal, but its ESR spin susceptibility above 150 K shows a weakly semiconducting behavior, presumably because during ESR measurements the sample cooling rate is slow hence allowing the disordered anions to readjust their positions. (ET)2CH (SO2CF3)2 is a 2D metal and undergoes a metal-to-insulator (MI) transition at 110 K due probably to a geometry change of the donor molecule layers. (ET)2C(SO2CF3)3 is a one-dimensional (1D) metal and undergoes an MI between 180 and 240 K, which is expected to be of charge density wave type.

A nonstoichiometric ternary vanadium sulfide, BaxV6S8 (x = 0.41−0.48), was synthesized. This solid-state compound crystallizes in the hexagonal space group P63/m (no. 176) with a = 9.2080(3) A, c = 3.3169(2) A, V = 243.55(2) A3, and Z = 1. The structure of this solid is similar to that of its parent compound V3S4, consisting of face-sharing, distorted octahedral VS6 units. A superconducting transition of the solid at approximately 2.5 K was observed. A computational analysis of the electronic structure using the extended Huckel method is presented.

Deuterium substitution of the ethylene end groups of the ET [ET: BEDT-TTF or bis(ethylenedithio)tetrathiafulvalene] electron-donor molecule results in a 0.25 K increase in the superconducting transition temperature of three molecular superconductors derived from this molecule. Simplistically speaking, this change in T c is in contradiction to that predicted by the electron–phonon coupling mechanism of the BCS theory. We suggest that the slight shortening of the C–D bond relative to the C–H bond, coupled with the recent findings of a large, negative uniaxial pressure derivative of T c in κ-(ET) 2 Cu(SCN) 2 and β ′′ -(ET) 2 SF 5 CH 2 CF 2 SO 3 , can explain this unique effect. Herein we report the first study of the effect of deuterium substitution on the superconducting transition temperature in a molecular-based superconductor in which the electron-donor molecules are packed in a β ′′ motif, viz., β ′′ -(ET) 2 SF 5 CH 2 CF 2 SO 3 . This compound is ideally suited for this study because it contains discrete (non-polymeric) anions, has a completely ordered structure, is indefinitely stable in air at room temperature, and is free from possible magnetic impurities. Substitution of the eight hydrogen atoms of the ET molecule by deuterium causes the T c of β ′′ -(ET) 2 SF 5 CH 2 CF 2 SO 3 to increase from 4.34±0.05 to 4.61±0.03 K. These values were determined by measuring several representative crystals from various parallel electrocrystallization experiments containing h 8 - or d 8 -ET that was prepared in parallel syntheses. This is the first example which demonstrates that the inverse (positive) isotope effect previously observed in κ-phase salts is also present in a β ′′ -phase superconductor.

We measured the polarized infrared reflectance of β‘-(ET)2SF5CF2SO3 as a function of temperature and analyzed the spin-exchange interactions of this salt by calculating the spin−orbital interaction energies between adjacent spin sites. The observed changes in vibrational properties below the 45 K transition support a weak lattice distortion in combination with a spin gap in this material. The spin-exchange interaction is predicted to occur primarily along the (−a +b) diagonal direction, in good agreement with the spectral data. In addition, notable frequency shifts of vibrational modes point to a second transition between 100 and 200 K. The low-lying electronic excitation in the stack direction (assigned as a charge-transfer feature based upon our electronic band structure calculations) shows distinct changes with temperature that are consistent with trends in the vibrational spectrum. The pattern of a high-temperature transition preceding the spin-Peierls transition is discussed in relation to other low-...

A new organic conductor, β″-(BEDT-TTF)2SF5CHFCF2SO3 [BEDT-TTF, hereafter abbreviated ET, refers to bis(ethylenedithio)tetrathiafulvalene], was prepared by electrocrystallization. The crystal structure of this salt was determined by single crystal X-ray diffraction at 298 and 150 K, its physical properties were examined by electrical resistivity, Raman spectroscopy and EPR measurements, and its electronic structure was calculated and compared with that of the analogous salt β″-(ET)2SF5CH2CF2SO3. Whereas β″-(ET)2SF5CHFCF2SO3 has disordered anions and undergoes a metal-to-insulator transition at ∼190 K, β″-(ET)2SF5CH2CF2SO3 has ordered anions and is semiconducting down to ∼100 K, metallic below ∼100 K, and superconducting below 5 K. At room temperature both β″-(ET)2SF5CHFCF2SO3 and β″-(ET)2SF5CH2CF2SO3 have similar electronic band structures and physical properties. When the temperature is lowered, each donor molecule stack becomes dimerized in both salts. However, the interdimer interaction within each donor stack nearly vanishes in β″-(ET)2SF5CHFCF2SO3, but remains substantial in β″-(ET)2SF5CH2CF2SO3.

We report the polarized infrared reflectance spectra, optical conductivity, and electronic band structure of metallic β‘ ‘-(ET)2SF5CHFSO3 and compare our results with those of the β‘ ‘-(ET)2SF5CH2CF2SO3 superconductor and the β‘ ‘-(ET)2SF5CHFCF2SO3 metal/insulator material. We discuss the electronic structure of these organic molecular solids in terms of band structure, many-body effects, and disorder. On the basis of spectral similarities between the superconductor and metallic salts and structural differences in the anion pocket of all three, we conclude that the unusual electronic excitations observed in the β‘ ‘-(ET)2SF5CHFCF2SO3 metal/insulator material are not caused by electron correlation but are due to disorder-related localization.

Salts of the donor molecule, bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET), with pentafluorothiomethylsulfonate (SF{sub 5}CX{sub 2}SO{sub 3}{sup -}, X = H or F) anions have been prepared. Three phases, {beta}'-(ET){sub 2}SF{sub 5}CH{sub 2}SO{sub 3}, {beta}'-(ET){sub 2}SF{sub 5}CF{sub 2}SO{sub 3}, and {beta}'-(ET){sub 2}SF{sub 5}CHFSO{sub 3} were obtained by electrocrystallization with the corresponding LiSF{sub 5}CX{sub 2}SO{sub 3} electrolytes. The structures of these salts were determined by single-crystal X-ray diffraction, and their physical properties were examined by electrical resistivity measurements as well as by ESR and Raman spectroscopy. The {beta}'-(ET){sub 2}SF{sub 5}CH{sub 2}SO{sub 3}, {beta}'-(ET){sub 2}SF{sub 5}CHFSO{sub 3} and {beta}'-(ET){sub 2}SF{sub 5}CF{sub 2}SO{sub 3} salts are considerably different in their crystal structures, physical properties, and electronic structures despite the similarity in the structures of the SF{sub 5}CX{sub 2}SO{sub 3}{sup -} (X = H, F) anions. The {beta}'-(ET){sub 2}SF{sub 5}CH{sub 2}SO{sub 3} salt has two kinds of ET donor molecules with considerably different charge densities. The electronic structure of {beta}'-(ET){sub 2}SF{sub 5}CHFSO{sub 3} has both one-dimensional (1D) and two-dimensional (2D) Fermi surfaces which are similar to those found in the organic superconductor {beta}'-(ET){sub 2}SF{sub 5}CH{sub 2}CF{sub 2}SO{sub 3}. The ESR data for the {beta}'-(ET){sub 2}SF{sub 5}CF{sub 2}SO{sub 3} salt indicate that it opens a spin gapmore » below 45 K. The differences in the physical properties of the three salts were analyzed by calculating the HOMO-HOMO interaction energies between nearest-neighbor ET molecules in their donor molecule layers.« less

A number of layered conducting BEDT-TTF, bis(ethylenedithio)tetrathiafulvalene, salts with heavily fluorinated organosulfonate anions have been prepared and characterized. Of particular interest are the salts containing SF 5 R SO 3 − anions, where R is a partially fluorinated aliphatic backbone. While structurally similar —the β″ packing type predominates—the ground state of these salts varies from superconducting in the case of β″-(BEDT-TTF) 2 SF 5 CH 2 CF 2 SO 3 [1] to insulating. Many of the salts with insulating ground states are metallic at room temperature, but charge localization and disproportionation over crystallographically non-equivalent sites occurs at low temperature. The organosulfonate group exhibits a propensity to bind to lithium ions, thus ternary salts incorporating Li + into the complex anion layer are often found. The fluorophilic effect in organofluorine compounds may be exploited to form salts where the conducting BEDT-TTF layers are separated by extremely bulky anion bilayers. The crystal structure of one such system, (BEDT-TTF) 3 [(CF 3 ) 2 CFC 2 H 4 SO 3 ] 4 (H 5 O 2 ) 2 , is described here.

A nonstoichiometric ternary vanadium sulfide, BaxV6S8 (x = 0.41−0.48), was synthesized. This solid-state compound crystallizes in the hexagonal space group P63/m (no. 176) with a = 9.2080(3) A, c = 3.3169(2) A, V = 243.55(2) A3, and Z = 1. The structure of this solid is similar to that of its parent compound V3S4, consisting of face-sharing, distorted octahedral VS6 units. A superconducting transition of the solid at approximately 2.5 K was observed. A computational analysis of the electronic structure using the extended Huckel method is presented.

The inverse deuterium isotope effect, previously found in κ-(BEDT-TTF) 2 Cu(NCS) 2 , is also found in two other BEDT-TTF based superconductors with different packing motifs and different types of anions. Remarkably, the magnitude of the isotope shift is essentially identical in all three superconductors, ca. +0.26 ±0.06 K. These results, when taken together with the recent results of Lang et al. on the uniaxial pressure derivatives of T c , suggest that the inverse isotope effect may not have a direct relationship to the pairing mechanism but instead is a reflection of the change in the internal lattice pressure.

The beta''-(BEDT-TTF)(2)CF(3)CH(2)SO(3) organic conductor has been synthesized by electrocrystallization. The crystal structure was determined by single-crystal X-ray diffraction and electronic properties examined. The polarized IR reflectance of beta''-(BEDT-TTF)(2)SF(5)CH(2)SO(3) and beta''-(BEDT-TTF)(2)CF(3)CH(2)SO(3), as well as Raman spectra of beta''-(BEDT-TTF)(2)CF(3)CH(2)SO(3) have been measured as a function of temperature. Both materials display charge-nonequivalence within the whole temperature range and unusual activation of vibronic modes with lowering the temperature. In particular, electron-molecular vibration coupling is involved in the activation of the nu(27) mode. The effect is discussed in terms of lattice dimerization involving hydrogen bonding between the anion layer and the conducting BEDT-TTF layer.