Effects of Chemical Pressure on Superconductivity in Electrochemically Intercalated (TMA)_yFe₂(Se_1−xS_x)₂ (TMA = Tetramethylammonium)

Abstract The β‐modification of FeSe, which has an anti‐PbO type structure, achieves superconductivity at 8 K without external doping or pressure and exhibits a nematic phase, which has been crucial for studies of unconventional pairing mechanisms. Although the critical temperature (T_c) in FeSe increases significantly with applied pressure, intercalation, and in thin films, the effect of chemical pressure by replacing selenium with sulfur in FeSe_1−xS_x has remained unclear. To investigate the effects of chemical pressure, we have synthesized FeSe_1−xS_x crystals (up to x = 0.52) and intercalated them with tetramethylammonium ions (TMA⁺). These results show that both T_c and unit cell volume decrease linearly with sulfur content in both host and intercalated (TMA)_yFe₂(Se_1‐xS_x)₂. The remarkable common rate of normalized decrease of T_c suggests that chemical pressure affects the electron‐doped intercalates in the same way as the host compounds. This result highlights the unique role of chemical pressure and electron doping in tuning superconductivity in FeSe systems and provides new insights into their complex behavior.