[en] A series of 1:1 C₆₀ Cycloaddition adducts, C₆₀A (A = anthracene, butadiene, cyclopentadiene, and methylcyclopentadiene), has been synthesized. The products are cleanly separated and characterized by use of TGA, 1H-NMR, IR, and mass spectrometry. Among these adducts, C₆₀(methylcyclopentadiene) showed the highest thermal stability and was doped with three equivalents of rubidium. The resulting Rb₃C₆₀(MeCp) is a semiconductor but can be thermally converted to the superconducting Rb₃C₆₀ through a retro-Diels-Alder reaction. A one-step doping process to prepare Rb₃C₆₀ crystals has been developed. The optimal doping condition occurs at ∼ 300 degrees C. High superconducting shielding fractions between 60 and 90% and sharp transition widths (ΔT_10--90 between 4 and 0.7 K) were measured for these samples

[en] A new approach to synthesis of organic superconductors has recently been pioneered which involves the use of large discrete molecular anions as the charge-compensating entities in these charge transfer salts. The organic electron-donor molecule bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET) has been electrocrystallized with the novel organometallic M(CF₃)₄^- (M=Cu, Ag, Au) anions in a variety of 1,1,2-trihaloethane solvents. Over 20 organic superconductors have been synthesized which can be described by (ET)₂M(CF₃)₄(1,1,2- trihaloethane). These solvated salts are shown to have highly anisotropic physical properties which can be tuned via modifications of each of their three molecular components: ET electron donor molecule, M(CF₃)₄^- anion, and neutral 1,1,2- trihaloethane solvent molecule. Superconductivity has also been observed in an ET salt containing the discrete SF₅CH₂CF₂SO₃^- anion with onset temperature near 5.2 K

[en] Since the initial discovery of organic superconductivity in 1979, a large number of organic superconductors have now been synthesized. However, the mechanism of electron-pairing in these novel superconductors has remained largely unresolved. Isotope effect studies constitute an important experimental tool for the investigation of whether or not the electron-pairing mechanism in organic superconductors is phonon-mediated, as in conventional superconductors. Recent isotope effect studies in the authors' laboratory, involving seven different isotopically labeled BEDT-TTF (or ET) derivatives, have demonstrated the following: (1) intramolecular phonon modes involving C double-bond C and Csingle bondS stretching vibrations in the ET donor molecule are not the dominant mediators of electron-pairing, and (2) in κ-(ET)₂Cu(NCS)₂, there exist two competing isotope effects--a normal mass effect, i.e., lowering of T_c upon isotopic labeling, when the ET molecular mass is increased by concurrent ¹³C and ³⁴S labeling, in addition to an inverse isotope effect upon deuterium labeling in ET. It is of great interest to investigate if there is an isotope effect when the charge-compensating anions, which are also located within the non-conducting layer in the superconducting cation-radical salts, are isotopically labeled. The existence of an isotope effect when the anions are labeled would be indicative of electron-pairing with the mediation of vibrational frequencies associated with the anions. In this paper, the authors present the results of the first isotope effect study in which isotopic labeling in the anion portion of κ-(ET)₂Cu(NCS)₂ is carried out. The authors find no isotope effect when the carbon and nitrogen atoms of the thiocyanate groups in the anion are replaced with ¹³C and ¹⁵N isotopes

[en] We determined the pressure dependence of the superconducting transition temperature, T_c, of the organic superconductor κ-(BEDT-TTF)₂Cu(SCN)₂ for three different isotopic compositions using helium as a pressure medium. These measurements demonstrated that, given identical measurement conditions, the pressure dependence of T_c is independent of the isotopic composition of the material. Assuming that these isotopically different materials have an identical unit cell compressibility, it is found that for all three materials T_c scales linearly with the quasi-two-dimensional unit cell area, and is thus inversely proportional to the quasi-two-dimensional carrier density in κ-(BEDT-TTF)₂Cu(SCN)₂

[en] We report detailed measurements of the interlayer magnetoresistance of the layered organic superconductor κ-(BEDT-TTF)₂Cu(SCN)₂ for temperatures down to 0.5 K and fields up to 30 T. The upper critical field is determined from the resistive transition for a wide range of temperatures and field directions. For magnetic fields parallel to the layers, the upper critical field increases approximately linearly with decreasing temperature. The upper critical field at low temperatures is compared to the Pauli paramagnetic limit, at which singlet superconductivity should be destroyed by the Zeeman splitting of the electron spins. The measured value is comparable to a value for the paramagnetic limit calculated from thermodynamic quantities but exceeds the limit calculated from BCS theory. The angular dependence of the upper critical field shows a cusplike feature for fields close to the layers, consistent with decoupled layers.

No abstract available

[en] Low temperature upper critical field studies have been carried out in a new organic superconductor βdouble_prime-(BEDT-TTF)₂SF₅CH₂CF₂SO₃. For field parallel to the superconducting layers, the upper critical field determined from transport measurements exceeds the BCS Pauli limit at low temperatures. The angular dependence of the resistive transition shows that the upper critical field can be best described by a quasi-two-dimensional model with a cusp near the field parallel to the plane direction

[en] We employed infrared spectroscopy along with complementary lattice dynamics and spin density calculations to investigate pressure-driven local structure distortions in the copper coordination polymer Cu(pyz)F₂(H₂O)₂. Here, pyz is pyrazine. Our study reveals rich and fully reversible local lattice distortions that buckle the pyrazine ring, disrupt the bc-plane O-H (hor_ellipsis) F hydrogen-bonding network, and reinforce magnetic property switching. The resiliency of the soft organic ring is a major factor in the stability of this material. Interestingly, the collective character of the lattice vibrations masks direct information on the Cu-N and Cu-O linkages through the series of pressure-induced Jahn-Teller axis switching transitions, although Cu-F bond softening is clearly identified above 3 GPa. These findings illustrate the importance of combined bulk and local probe techniques for microscopic structure determination in complex materials.

[en] The electrolyte additive 2-(pentafluorophenyl)-tetrafluoro-1,3,2-benzodioxaborole (PFPTFBB, 1) was found to have a reversible redox potential at 4.43 V vs. Li⁺/Li. This compound can function as an overcharge protection additive as well as anion receptor for lithium-ion batteries. It has drawn a great deal of interest from industry, but its use in relatively large quantities is limited by the production challenges of tetrafluorocatechol (TFC, 3), which is the key starting chemical for the synthesis of PFPTFBB. As part of a continuous effort in our research toward improving the safety of lithium-ion batteries, we have performed the synthesis of TFC and optimized its synthesis process. The X-ray single-crystal structures of TFC and the intermediate product 5,6,7,8-tetrafluoro-1,4-benzodioxane (4) during the process of PFPTFBB synthesis are reported for the first time. Also presented is the lithium ion cell performance of PFPTFBB as redox shuttle in various electrolyte systems.

[en] Crystallographic lattice compression and magnetic variations in the quasi-1-D ferromagnet 2-(4,5,6,7-tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H -imidazole-3-oxide-1-oxyl (F4BImNN) were studied under hydrostatic pressure for polycrystalline samples. The crystallographic c-axis - along which F4BImNN forms hydrogen bonds - was compressed by 3% at 10 kbar and by 4% at 17.8 kbar. The overall lattice volume contracts by 12% from ambient pressure to 17.8 kbar. The axis of the hydrogen bonded chain propagation is compressed, since the bent F4BImNN hydrogen bonds accommodate geometric compression. The magnetic susceptibility measured over 1.8-300 K showed an increase in ferromagnetic exchange interactions as pressure increased. Curie-Weiss 1/χ vs T analyses showed an increase in Weiss constant from (+)10.4 K to (+)15.3 K from ambient pressure to 9.85 kbar. Fitting of the χT vs T data indicated predominantly 1-D Heisenberg chain type behavior at all pressures, increasing from J/k = (+)19 K at room pressure to (+)25 K at 4.77 kbar and (+)30 K at 9.85 kbar. The increase in exchange strength is attributed to pressure-increased overlap of spin orbitals in the hydrogen bonded chains, which favors 1-D ferromagnetic interaction. There was no indication of a phase change, either crystallographically or in discontinuities of the magnetic behavior.