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 s hows 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 po sitive charge localised on one of the two crystallographically indepen dent BEDT-TTF at room temperature is equally distributed at 10 K and f urthermore that the CuBr42- remains square planar, contrary to the hyp othesis of a Jahn-Teller distortion suggested by EPR. The pressure dep endent crystallographic study (0 - 15 kbar) may explain the dramatic e ffect of applying pressure on organic crystals. The intermolecular emp ty spaces are strongly reduced at 10 kbar explaining the increase of c onductivity. The crystallographic results require the physical propert ies of this salt at low temperature to be re-examined.