New Organic Superconductors with an Incommensurate Anion Lattice Consisting of Polyhalide Chains (MDT-TSF)<i>X</i><i>_y</i> (MDT-TSF = Methylenedithiotetraselenafulvalene; <i>X</i> = Halogen; <i>y</i> = 1.27−1.29)

The title radical cation salts of MDT-TSF (methylenedithiotetraselenafulvalene) with polyhalide anions prepared by electrocrystallization in the presence of various tetrabutylammonium trihalides show high conductivity at room temperature (>103 S cm-1) and become superconductors at below 4.0−5.5 K under ambient pressure. Their crystal structures elucidated by X-ray structural analysis are characterized by quasi two-dimensional conducting sheets consisting of uniform donor stacks. Their X-ray oscillation photographs reveal the existence of an incommensurate anion lattice originating from the infinite polyhalide chains. The chemical composition of these salts, as determined on the basis of the X-ray photographs together with the combustion elemental analyses, is (MDT-TSF)X1.27-1.29 (X = halogen). Considering that the most probable charged unit in the polyhalide chain is the trihalide (X3-), the charge-transfer degree on each donor molecule is estimated to be nonstoichiometric, 0.42−0.43.