J. Phys. IV France 10 (2000) Pr3-147-Pr3-151
βCO-(ET)2I3 : Crystallographic evidence for the co-existence of ordered β*-(ET)2I3 (TC 8 K) and disordered β-(ET)2I3 (TC 2 K)H. Müller1, D. Madsen1, 2, A.N. Fitch1, S. Wanka3 and J. Wosnitza3
1 European Synchrotron Radiation Facility, BP. 220, 38043 Grenoble, France
2 Centre for Crystallographic Studies, Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark
3 Physikalisches Institut der Universität Karlsruhe, Engesserstr. 7, 76128 Karlsruhe, Germany
The crystal structures of microcrystalline βCO-(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 βCO-(ET)2I3 at 293 K is similar to that of the known superconductor β-(ET)2I3 (Tc = 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 βCO-(ET)2I3-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 Tc-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 (TC 8 K) and disordered β-(ET)2I3 TC= 1.4 - 1.8 K) in the title compound.
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