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

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.