Superconductivity at 5.2 K in an Electron Donor Radical Salt of Bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) with the Novel Polyfluorinated Organic Anion SF₅CH₂CF₂SO₃^-

Our search for new organic superconductors has led us to explore the suitability of large polyfluorinated anions, in analogy to these perfluorinated organometallic complex anions. In this paper, we report the discovery of superconductivity (diamagnetic onset transition temperature of 5.2 K) at ambient pressure in {beta}-(ET){sub 2}SF{sub 5}CH{sub 2}CF{sub 2}SO{sub 3}, along with the synthesis and the crystal and band electronic structures of this novel salt. Small single crystals of the title compound were synthesized by the electrocrystallization method, with the use of a mixture of LiSF{sub 5}CH{sub 2}CF{sub 2}SO{sub 3} and 12-crown-4 in 1,1,2-trichloroethane as a electrolyte. An initial current density of 0.20 {mu}A/cm{sup 2} was applied through the platinum wire electrodes and then increased to 0.41 {mu}A/cm{sup 2} after 1 week, at which point crystallization commenced. The crystal growth was continued for 35 days. Inferior crystals of the same composition and crystal structure, but with lower T{sub c} values, were formed when tetrdydrofuran was used as a solvent. {beta}-(ET){sub 2}SF{sub 5}CH{sub 2}CF{sub 2}SO{sub 3} is the first example of an organic superconductor where not only the radical cation but also the charge-compensating anion consists of an organic molecule. The vast majority of organic superconductors contain diamagnetic metal complexmore » anions, while the remainder of the previously reported superconducting salts contain small inorganic anions such as polyhalides. 20 refs., 2 figs.« less