Multistability in a BEDT-TTF Based Molecular Conductor

The low dimensional organic conductor (BEDT-TTF)₂Br_1.3I_1.1Cl_0.6 [BEDT-TTF = bis(ethylenedithio)tertathiafulvalene] is shown to be a unique molecular solid that exists in three crystalline polymorphic forms (α‘-, α‘ ‘‘-, β‘ ‘-phase) and, surprisingly, is able to adopt the same metal-like β‘ ‘-phase at both low (T < 185 K) and high (T > 395 K) temperatures. Several crystals of the α‘- and α‘ ‘‘-phases have been studied using three different techniques:  dc-conductivity measurements, ESR spectroscopy, and X-ray diffraction analysis. All these techniques show the existence of the reversible semiconductor ↔ metal (α‘ ‘‘ ↔ β‘ ‘) phase transition at both high and low temperatures as well as the α‘ ↔ α‘ ‘‘ phase transition at high temperatures. The phase transitions of these polymorphs are characterized by huge hysteresis and dramatic changes in the transport and magnetic properties. Based on ab initio calculations, it is suggested that dipole−dipole interactions can play a key role in the rich polymorphism of this molecular solid.