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Superconductivity in iron selenides has experienced a rapid growth, but notwithout major inconsistencies in the reported properties. Foralkali-intercalated iron selenides, even the structure of the superconductingphase is a subject of debate, in part because the onset of superconductivity isaffected much more delicately by stoichiometry and preparation than in cuprateor pnictide superconductors. If high-quality, pure, superconductingintercalated iron selenides are ever to be made, the intertwined physics andchemistry must be explained by systematic studies of how these materials formand by and identifying the many coexisting phases. To that end, we preparedpure K_2Fe_4Se_5 powder and superconductors in the K_xFe_{2-y}Se_2 system, andexamined differences in their structures by high-resolution synchrotron andsingle-crystal x-ray diffraction. We found four distinct phases: semiconductingK_2Fe_4Se_5, a metallic superconducting phase K_xFe_2Se_2 with x ranging from0.38 to 0.58, an insulator KFe_{1.6}Se_2 with no vacancy ordering, and anoxidized phase K_{0.51(5)}Fe_{0.70(2)}Se that forms the PbClF structure uponexposure to moisture. We find that the vacancy-ordered phase K_2Fe_4Se_5 doesnot become superconducting by doping, but the distinct iron-rich minority phaseK_xFe_2Se_2 precipitates from single crystals upon cooling from above thevacancy ordering temperature. This coexistence of metallic and semiconductingphases explains a broad maximum in resistivity around 100 K. Further studies tounderstand the solubility of excess Fe in the K_xFe_{2-y}Se_2 structure willshed light on the maximum fraction of superconducting K_xFe_2Se_2 that can beobtained by solid state synthesis.

Phase relations in K_xFe_{2-y}Se_2 and the structure of superconducting K_xFe_2Se_2 via high-resolution synchrotron diffraction