β"-(CNB-EDT-TTF)4BF4; Anion Disorder Effects in Bilayer Molecular Metals
The preparation and characterization of new salts based on the dissymmetrical TTF derivative CNB-EDT-TTF (cyanobenzene-ethylenedithio-tetrathiafulvalene) and BF 4 − anions, are reported. Depending on the electrocrystallization conditions salts with different stoichiometries, (CNB-EDT-TTF)BF 4 and β"-(CNB-EDT-TTF) 4 BF 4 , can be obtained. The 1:1 salt is an electrical insulator isostructural to the ClO 4 analogue previously described. The 4:1 salt is a new member of the family of 2D metals of
... y of 2D metals of this donor with different small anions X, (CNB-EDT-TTF) 4 X, characterized by a bilayer arrangement of the donors and it was obtained in a monoclinic polymorph with a β"-type donor packing pattern. The small anions in this compound are severely disordered between the donor bilayers, which present slightly larger lattice parameters than the isostructural ClO 4 analogue. Both electrical conductivity and thermoelectric power measurements in single crystals denote metallic properties as predicted by electronic band structure calculations. As a consequence of the anion disorder the metallic regime of the electrical conductivity denotes electronic localization effects with a progressive increase of resistivity below~25 K. Because of the larger lattice parameters the intermolecular interactions and electronic bandwidth are decreased compared to other (CNB-EDT-TTF) 4 X salts. The large and positive thermoelectric power S of this compound (~110 µV/K in the range 100-330 K) and its electrical conductivity σ = 20 S/cm at room temperature lead to a power factor S 2 σ = 24 µW/K 2 m, quite large among molecular conductors, placing these compounds as potential candidates for thermoelectric materials.