New Organic Superconductors with an Incommensurate Anion Lattice Consisting of Polyhalide Chains (MDT-TSF)Xy (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 (>10 3 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.