  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
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 DFT 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.
