Observation of superconductivity at 30 K~46 K in AxFe2Se2 (A = Li, Na, Ba, Sr, Ca, Yb, and Eu)
New iron selenide superconductors by intercalating smaller-sized alkali metals (Li, Na) and alkaline earths using high-temperature routes have been pursued ever since the discovery of superconductivity at about 30 K in KFe2Se2, but all have failed so far. Here we demonstrate that a series of superconductors with enhanced Tc=30~46 K can be obtained by intercalating metals, Li, Na, Ba, Sr, Ca, Yb, and Eu in between FeSe layers by the ammonothermal method at room temperature. Analysis on their powder X-ray diffraction patterns reveals that all the main phases can be indexed based on body-centered tetragonal lattices with a~3.755-3.831 Å while c~15.99-20.54 Å. Resistivities show the corresponding sharp transitions at 45 K and 39 K for NaFe2Se2 and Ba0.8Fe2Se2, respectively, confirming their bulk superconductivity. These findings provide a new starting point for studying the properties of these superconductors and an effective synthetic route for the exploration of new superconductors as well.