Ambient-Pressure Organic Superconductor κ-(ET)2Ag(CN)[N(CN)2] Formed with Polymeric Silver(I) Complex Anion

We obtained a new cation radical salt of bis(ethylenedithio)tetrathiafulvalene (ET) with a κ-type packing motif, κ-(ET)2Ag(CN)[N(CN)2], by electrooxidation. In the salt, each ET layer is separated by polymeric {Ag(CN)[N(CN)2]−}∞ layers, which are composed of Ag(I) ions trigonally coordinated to form an infinite zigzag chain of –NC–Ag–NC–Ag– with pendant dicyanamides. The salt undergoes a superconducting transition with a midpoint critical temperature (Tc) of 6.6 K and an onset Tc of 7.2 K, thereby indicating the highest Tc among ET superconductors formed with a polymeric Ag(I) complex anion.