Electron spin resonance, infrared spectroscopic, and molecular packing studies of the thermally induced conversion of semiconducting α- to superconducting α1-(BEDT-TTF)2I3

The semiconducting α- to superconducting α1-(ET)2I3 (Tc ∼ 7.2 K) thermal conversion was studied by use of the temperature dependence of the ESR peak-to-peak line width and derived spin susceptibility. The term ET is an abbreviation for BEDT-TTF, i.e., bis(ethylenedithio)tetrathiafulvalene. The phase transition occurred above 340 K and was not fully complete during the time span of the ESR study. Fourier transfer-infrared (FT-IR) reflectance measurements were also performed on α1-(ET)2I3, which was obtained by heating α-(ET)2I3 at 343 K in an ethylene glycol bath for 4 days. The reflectance spectrum of the thermally converted non-single-crystalline α1(ET)2I3 exhibits an absorption dip, which is due to the C-C-H hor-ellipsis X interactions around the anion cavity, at ∼ 1280 cm-1, just as in the case of all superconducting β-(ET)2X (X = I3-, IBr2-, AuI2-) salts, thereby suggesting that α1-(ET)2I3 has a β-like structure. Structural factors associated with the thermal conversion, α- to α1-(ET)2I3, were also examined by analysis and comparison of the crystal structures of α-(ET)2I3 and the unique superconducting phase, β*-(ET)2I3 (Tc ∼ 8 K). 22 refs., 5 figs