Electrochemical Synthesis and Crystal Structure of the Organic Ion Intercalated Superconductor (TMA)_0.5Fe₂Se₂ with <i>T</i>_c = 43 K

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