β"-(CNB-EDT-TTF)4BF4; Anion Disorder Effects in Bilayer Molecular Metals

Sandra Rabaça, Sandrina Oliveira, Vasco Gama, Isabel Santos, Gonçalo Oliveira, Elsa Lopes, Enric Canadell, Manuel Almeida
2018 Crystals  
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
more » ... 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.
doi:10.3390/cryst8040142 fatcat:aqhunkltsnglvms4jalm3jsram