New organic superconductors with an incommensurate anion. Lattice consisting of polyhalide chains (MDT-TSF)X_y (MDT-TSF = methylenedithiotetraselenafulvalene; X = halogen; y = 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)X_1.27-1.29 (X = halogen). Considering that the most probable charged unit in the polyhalide chain is the trihalide (X ₃^-), the charge-transfer degree on each donor molecule is estimated to be nonstoichiometric, 0.42-0.43.