Electrochemical synthesis and crystal structure of the organic ion intercalated superconductor (TMA)0.5Fe2Se2 with Tc = 43 K

Intercalation of organic cations in superconducting iron selenide can significantly increase the critical temperature (Tc). We present an electrochemical method using β-FeSe crystals (Tc ≈ 8 K) floating on a mercury cathode to intercalate tetramethylammonium ions (TMA+) quantitatively to obtain bulk samples of (TMA)0.5Fe2Se2 with Tc = 43 K. The layered crystal structure is closely related to the ThCr2Si2-type with disordered TMA+ ions between the FeSe layers. Although the organic ions are not detectable by X-ray diffraction, packing requirements as well as first-principle density functional theory calculations constrain the specified structure. Our synthetic route enables electrochemical intercalations of other organic cations with high yields to greatly optimize the superconducting properties and to expand this class of high-Tc materials.