Phase diagram of (Li1-xFex)OHFeSe: a bridge between iron selenide and arsenide superconductors

Previous experimental results have shown important differences between iron selenide and arsenide superconductors, which seem to suggest that the high temperature superconductivity in these two subgroups of iron-based family may arise from different electronic ground states. Here, we report the complete phase diagram of a newly synthesized superconducting (SC) system (Li1-xFex)OHFeSe with a similar structure to FeAs-based superconductors. In the non-SC samples, an antiferromagnetic (AFM) spin-density-wave (SDW) transition occurs at ~127 K. This is the first example to demonstrate such an SDW phase in FeSe-based superconductor system. Transmission electron microscopy (TEM) shows that a well-known sqrr(5) x sqrr(5) iron vacancy ordered state, resulting in an AFM order at ~ 500K in AyFe2-xSe2 (A = metal ions) superconductor system, is absent in both non-SC and SC samples, but a unique superstructure with a modulation wave vector q=1/2(1, 1, 0), identical to that seen in SC phase of KyFe2-xSe2, is dominant in the optimal SC sample (with an SC transition temperature Tc = 40 K). Hence, we conclude that the high-Tc superconductivity in (Li1-xFex)OHFeSe stems from the similarly weak AFM fluctuations as FeAs-based superconductors, suggesting a universal physical picture for both iron selenide and arsenide superconductors.