beta(CO)-(ET)(2)I-3: Crystallographic evidence for the co-existence of ordered beta*-(ET)(2)I-3 (T-c approximate to 8 K) and disordered beta-(ET)(2)I-3 (T-c approximate to 2 K)

The crystal structures of microcrystalline beta(CO) - (ET)(2)I-3 were deter mined at temperatures of 293, 200, 130; 100, and 35 K from X-ray single cry stal diffraction data employing synchrotron radiation. The structure of bet a(CO)-(ET)(2)I-3 at 293 K is similar to that of the known superconductor be ta-(ET)(2)I-3 (T-c = 1.4 - 2 It). By contrast to beta-(ET)(2)I-3, the title compound was found to be slightly iodine-deficient and is better described by the formula beta(CO)-(ET)(2)I3-x (x = 0.014). At 293 K, one ethylene gr oup of the ET cations is disordered with the eclipsed conformation prevaili ng over the staggered conformer,while at 200 K the occupancies of both arra ngements are equal. Upon cooling to 130 K and below, about 60% of the ethyl ene groups adopt a staggered conformation, thus providing clear-cut evidenc e for a temperature-induced, partial ordering process. Ac susceptibility me asurements show a T-c-onset at 8 K followed by a broad transition and a sec ond superconducting transition at around 2 K. The observation of both parti al end group ordering and twofold superconducting transition is interpreted as co-existence of ordered beta*-(ET)(2)I-3 (T-c approximate to 8 K) and d isordered beta-(ET)(2)I-3 (T-c = 1.4 - 1.8 K) in the title compound.