The discoveries of superconductivity at about 30 K in A_xFe_2-ySe₂ (A = K, Cs, Rb, Tl/Rb, Tl/K)^[1-5] have stimulated significant interest for the remarkably distinct structural and physical characteristics displayed in these metal-intercalated iron selenide superconductors. However, the presence of phase separations in the materials severely thwarts the attempts to unambiguously identify the superconducting phases and to understand the underlying mechanisms. Many efforts have been devoted to obtain phase-pure or superconducting-dominated single crystals by high-temperature routes and not succeeded. New strategies are needed to overcome this key problem. Here we report that (Na_xK_y)Fe_zSe₂ crystals are successfully prepared in NaCl/KCl flux at low temperatures~720 °C. A reduction reaction that converts K in KCl into metal K by metal Na plays a crucial role in the growth of crystals. It is found that K is more preferred than Na to enter in between FeSe layers and forms the superconducting phase. Thus-obtained superconducting crystals contain more superconducting phase in volume fraction than those by high-temperature routes and exhibit new features in transport property. Our results provide a new effective synthetic route for preparing quality crystals of iron selenide superconductors.

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