Structural properties of the superconducting salt (BEDT-TTF)(3)Cl-2 centerdot (H2O)(2) at low temperatures

The X-ray crystal structure of (BEDT-TTF)(3)Cl-2 . (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 resist ivity data at ambient pressure, and the pressure induced superconductivity (T-c = 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 t hree independent BEDT-TTF molecules. The motion can be considered as a dime rization of the BEDT-TTF along the a-axis. The thermal expansion tensor in the range 12 to 295 Ii is anisotropic and reveals no sharp anomaly. The sys tem remains two dimensional at all temperatures, in agreement with the elec trical conductivity measurements. (C) 1999 Academic Press.