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

Abstract

Electrocrystallization from solutions of the dissymmetrical ET derivative cyanobenzene-ethylenedithio-tetrathiafulvalene (CNB-EDT-TTF) in the presence of triiodide I affords two different polymorphs (β″ and κ) with the composition (CNB-EDT-TTF)I, 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″, 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″, 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/ρ 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.

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

“…This difference is certainly due to the severe anion disorder and it corresponds to a not so tight packing of the molecules in the layers. The donor bilayer structure is due to a network of bifurcated C-N … H interactions in an efficient combination of R 2 2(10) and R 2 2(8) synthons between coplanar molecules (Figure 3a) with contact distances (N1···H3 at 2.624 Å, N1···H5 at 2.505 Å) comparable to those previously found in β"-(CNB-EDT-TTF)4X salts [1][2][3]. However the stacking distance between successive planes of molecules interconnected by R 2 2(10) and R 2 2(8) synthons is slightly larger in this BF4 salt.…”

“…However the stacking distance between successive planes of molecules interconnected by R 2 2(10) and R 2 2(8) synthons is slightly larger in this BF4 salt. The three in β"-(CNB-EDT-TTF) 4 X salts [1][2][3]. However the stacking distance between successive planes of molecules interconnected by R 2 2 (10) and R 2 2 (8) synthons is slightly larger in this BF 4 salt.…”

“…The donor bilayer structure confers to these salts unusual metallic properties associated to high band filling, large electronic effective masses and quasi degenerated 2D Fermi surfaces, making the study of this type of bilayer metallic systems quite attractive. However the range of stability of the (CNB-EDT-TTF) 4 X salts was found so far restricted to linear, tetrahedral or octahedral anions of size comparable to ClO 4 − , PF 6 − , and I 3 − , and for instance with a larger anion such as FeBr 4 − , a salt of different stoichiometry (CNB-EDT-TTF) 2 FeBr 4 and distinct structure is obtained instead [4]. These findings directed our attention towards exploring new CNB-EDT-TTF salts with comparable small anions and in this contribution we report the salts obtained with the smaller tetrahedral anion BF 4 − .…”

“…All valence electrons were taken into account in the calculations and the basis set consisted of Slater-type orbitals of double-ζ quality for C and N 2s and 2p, S 3s and 3p and of single-ζ quality for H1s. The ionization potentials, contraction coefficients and exponents were taken exactly as in our previous work for (CN-BEDT-TTF) 4 I 3 [2]. Because of the different crystallographic cells the four different HOMO···HOMO intralayer interactions were defined as in the previous work on disordered (CN-BEDT-TTF) 4 I 3 [2]: I, interaction along the stacks; II, lateral π-type interaction; III, interaction along the step-chains and IV, head to head interlayer interaction .…”

“…This bilayer arrangement is promoted by an effective arrangement of hydrogen bond dimeric C-N···H-C interactions involving the cyano groups, as a combination of R 2 2 (10) and R 2 2 (8) synthons in a two-dimensional network. These (CNB-EDT-TTF) 4 X compounds have been obtained in several polymorphic phases associated with different donor packing patterns or anion ordering schemes [2,3] and for κ-type packing pattern some of these 2D metals have shown superconductivity at low temperatures [2]. [2,3] and for κ-type packing pattern some of these 2D metals have shown superconductivity at low temperatures [2].…”