Crystal structure and electrical conductivity of α‴-[BEDT-TTF]12[Cu2Br4]3 (BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene)

A new non-integer valence salt based on bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) with a dianionic dinuclear cupper(I) complex, α‴-[BEDT-TTF]12(CuI2Br4)3, was selectively synthesized by a diffusion of BEDT-TTF in tetrahydrofurane (THF) and a mixture of CuIIX2 (X=Br or Cl) in methanol. An alternating stacking arrangement of the BEDT-TTF layers and inorganic layers of the CuI2Br42- anions is formed, where the BEDT-TTF layer involves six crystallographically independent molecules. Overlap integral calculation among BEDT-TTF donors in the salt suggests the existence of two-dimensional interactions among the donors. The compound exhibits semiconducting behavior in the wide range of temperature of room temperature to 2 K (ρr.t.=0.3 Ω cm, Ea=0.08 eV in a crystal) without structural phase transition at ambient pressure.