βCO-(ET)2I3: Co-existence of ordered β-(ET)2I3 (Tc≈8 K) and disordered β-(ET)2I3 (Tc≈2 K)

The crystal structures of β CO -(ET) 2 I 3 were determined at temperatures of 293, 200, 130, 100, and 35 K from X-ray single crystal diffraction data. At 293 K, one ethylene group of the ET cations is disordered with the eclipsed conformation prevailing over the staggered conformer; at 200 K the occupancies of both arrangements are equal. Upon cooling to 130 K and below, about 60 % of the ethylene groups adopt a staggered conformation, thus providing evidence for a temperature-induced, partial ordering process. Kevwords: Organic superconductors, X-ray diffraction. BEDT-TTF.