Structural Properties of the Superconducting Salt (BEDT-TTF)3Cl2·(H2O)2 at Low Temperatures

The X-ray crystal structure of (BEDT-TTF)3Cl2·(H2O)2 [BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene] has been determined at 190, 130, and 10 K. The broad metal–insulator transition (100–160 K) observed in the resistivity data at ambient pressure, and the pressure induced superconductivity (Tc=5 K at 1.2 GPa) can be associated with temperature-dependent motion of the molecules within the unit cell and charge redistribution among the three independent BEDT-TTF molecules. The motion can be considered as a dimerization of the BEDT-TTF along the a-axis. The thermal expansion tensor in the range 12 to 295 K is anisotropic and reveals no sharp anomaly. The system remains two dimensional at all temperatures, in agreement with the electrical conductivity measurements.