Electronic band structure and superconducting transition of κ-(BEDT–TTF) 2 I 3

Crystals of κ-(BEDT–TTF)2I3 have been prepared electrochemically from a tetrahydrofuran (THF) solution containing the mixed supporting electrolytes {[(C4H9)4N]I3(95%)–[(C4H9)4N]AuI2(5%)} and also from a THF solution without [(C4H9)4N]Aul2. A resistivity drop indicating a superconducting transition was observed at 4.2 K (mid-point). The Tc decreased with increasing pressure (dTc/dP=–0.8 K kbar–1). The resistivity exhibited an anomaly around 170 K, which was coupled with the structural phase transition accompanied by the change of the space group (P21/c→P21). The crystal structure was determined at 295, 150 and 10 K. The extinction rule of the X-ray intensity data collected at 295 K confirmed the space group to be P21/c as reported previously. The (h 0 l) reflections at 150 and 10 K revealed the absence of c-glide symmetry. The low-temperature crystal structure analyses showed conformational change of the ethylene groups. The low-temperature band structure calculation based on a simple extended Hückel approximation gave Fermi surfaces consistent with de Haas–van Alphen experiments.