Low temperature crystal structure of the organic metal ([2H8]BEDT–TTF)4Cl2·6D2O [BEDT–TTF=bis(ethylenedithio)tetrathiafulvalene

Abstract

([2H8 ]BEDT–TTF)4Cl2 ·6D2O [BEDT–TTF=bis(ethylenedithio)tetrathiafulvalene] exhibits a transition from semimetal to semiconductor at T=160 K (Rosseinskyet al., J. Mater. Chem., 1993, 3, 801). This electronic transition is accompanied by a structural transition that is characterized by the reversible appearance of superstructure reflections corresponding to the doubling of the cell parameter b and a change in space group from Pcca to Pbcn. The crystal structures are very similar above and below T and the calculated intermolecular transfer integrals scarcely change. In contrast, from the difference in intramolecular bond lengths, it is clear that the two crystallographically independent BEDT–TTF molecules carry different charges at low temperature, suggesting that a degree of ionicity arising in the BEDT–TTF layer is responsible for the change in electrical behaviour.