Polymorphism and Superconductivity in Bilayer Molecular Metals (CNB-EDT-TTF)₄I₃

Electrocrystallization from solutions of the dissymmetrical ET derivative cyanobenzene-ethylenedithio-tetrathiafulvalene (CNB-EDT-TTF) in the presence of triiodide I₃^- affords two different polymorphs (β″ and κ) with the composition (CNB-EDT-TTF)₄I₃, both with a bilayer structure of the donors. These polymorphs differ in the packing patterns (β″- and κ-type) of the donor molecules in each layer, in both cases with bifurcated C-N···H interactions effectively coupling head-to-head donor molecules between layer pairs. Two β″ polymorphs can be obtained with different degrees of anionic ordering. In one disordered phase, β″_d, with a smaller unit cell, the triiodide anions are disordered over two possible positions in a channel between the donor bilayers, while in the ordered phase, β″_o, the triiodide anions occupy only one of those positions in this channel, leading to the doubling of the unit cell in the layer plane. These results for β″ phases contrast with the κ polymorph previously reported, for which weaker disorder of the triiodide anions, over two possible orientations with 94 and 6% occupation factors, was observed. While the β″ polymorphs remains metallic down to 1.5 K with a ρ_300K/ρ_4K resistivity ratio of 250, the κ polymorph presents a much smaller resistivity ratio in the range of 4-10 and superconductivity with an onset temperature of 3.5 K.