Temperature and pressure dependence of the crystal structure of the magnetic molecular conductor: (BEDT-TTF)(3)CuBr4

The magnetic molecular semiconductor (BEDT-TTF)(3)CuBr4 (BEDT-TTF: bis(ethylenedithio)-tetrathiafulvalene) exhibits a phase transition at 60 K accompanied by an abrupt fall in magnetic susceptibility. It also shows the largest increase of conductivity with pressure of all organic conductors with a transition to a metallic state at about 5 kbar. The temperature dependent structural study (10 - 298 K) shows that the positive charge localised on one of the two crystallographically independent BEDT-TTF at room temperature is equally distributed at 10 K and furthermore that the CuBr42- remains square planar, contrary to the hypothesis of a Jahn-Teller distortion suggested by EPR. The pressure dependent crystallographic study (0 - 15 kbar) may explain the dramatic effect of applying pressure on organic crystals. The intermolecular empty spaces are strongly reduced at 10 kbar explaining the increase of conductivity. The crystallographic results require the physical properties of this salt at low temperature to be re-examined.