A series of strongly one-dimensional organic metals with strictly uniform stacks: (o-DMTTF)2X (X = Cl, Br, I)

Abstract

The electrocrystallization of o-dimethyl-tetrathiafulvalene in the presence of halide anions affords 2:1 salts formulated as [o-DMTTF]2 X, (X = Cl, Br, I). The three salts are isomorphous, crystallizing in the tetragonal system, space group I42d, with the halide anion on the -4 site and the o-DMTTF molecule along a two-fold axis, forming strictly uniform stacks along c, and is further stabilized by C-H...X hydrogen bonds. Detailed physical investigations were performed on the bromide salt. It exhibits a metallic behaviour down to T(MI) = 50 K, while application of pressure moves the metal insulator transition to 39 K at 7.9 kbar. Band-structure calculations indicate a strong one-dimensional character, also confirmed by the anisotropy ratio of the conductivity, sigma / sigma = 500, as determined by two independent methods (four-contact, low-frequency transport measurements and analysis of the anisotropy of the Dysonian EPR line). The singular temperature dependence of the g tensor and the EPR line width at the phase transition suggests charge ordering.

Phase diagram of the correlated quarter-filled-band organic salt series (o-DMTTF)2X(Foury-Leylekian1, Auban-Sensier2, Coulon3 et al. 2011Phys. Rev. B

“…The striking difference is a much larger Curie component for the Br salt. As previously mentioned, 51 this extrinsic signal quickly becomes the dominant contribution to the ESR signal below 50 K. In this context, the shift of the g factor previously reported (Fig. 6 in Ref.…”

“…51 In the insulating state, the insert of Fig. 7 shows that S varies linearly with 1/T, as expected for an insulator from which we can deduce the gaps TEP = 260, 365, and 33 K for the Cl, Br, and I salts, respectively.…”

“…5 of Ref. 51. With respect to the data of the Cl salt, both kinds of linewidths of the Br salt are larger, and the minimum value is always found along the perpendicular direction: in this case if a crossover exists, it should be located above 300 K. Low-temperature data for the transverse direction are shown in Fig.…”

“…51.T h e typical sizes of the needles used were about 0.05-0.2 × 0.05-0.2 × 1-2 mm 3 along the a-, b-, and c-axes for all types of experiments. The measurements were performed on different samples issued from the same batches.…”

“…It appears important to revisit the physics of this family, first synthesized in the early 1980s, 50 in particular due to its original and very high-symmetry space group I42d. 51 This high symmetry is provided by the central position of halogen atoms, X = Cl, Br, I, leading to a strictly uniform stacking and a very weak coupling between first-neighbor orthogonal stacks (Fig. 1).…”