β_CO-(ET)₂I₃ : Crystallographic evidence for the co-existence of ordered β^*-(ET)₂I₃ (T_C 8 K) and disordered β-(ET)₂I₃ (T_C 2 K)

Abstract
The crystal structures of microcrystalline β_CO-(ET)₂I₃ were determined at temperatures of 293, 200, 130, 100, and 35 K from X-ray single crystal diffraction data employing synchrotron radiation. The structure of β_CO-(ET)₂I₃ at 293 K is similar to that of the known superconductor β-(ET)₂I₃ (T_c = 1.4 - 2 K). By contrast to β-(ET)₂I₃, the title compound was found to be slightly iodine-deficient and is better described by the formula β_CO-(ET)₂I_3-x (x = 0.014). At 293 K, one ethylene group of the ET cations is disordered with the eclipsed conformation prevailing over the staggered conformer, while 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 clear-cut evidence for a temperature-induced, partial ordering process. Ac susceptibility measurements show a T_c-onset at 8 K followed by a broad transition and a second superconducting transition at around 2 K. The observation of both partial end group ordering and twofold superconducting transition is interpreted as co-existence of ordered β^*-(ET)₂I₃ (T_C 8 K) and disordered β-(ET)₂I₃ T_C= 1.4 - 1.8 K) in the title compound.