Phase relations in K _xFe _2-ySe ₂ and the structure of superconducting K _xFe ₂Se ₂ via high-resolution synchrotron diffraction

Superconductivity in iron selenides has experienced a rapid growth, but not without major inconsistencies in the reported properties. For alkali-intercalated iron selenides, even the structure of the superconducting phase is a subject of debate, in part because the onset of superconductivity is affected much more delicately by stoichiometry and preparation than in cuprate or pnictide superconductors. If high-quality, pure, superconducting intercalated iron selenides are ever to be made, the intertwined physics and chemistry must be explained by systematic studies of how these materials form and by and identifying the many coexisting phases. To that end, we prepared pure K ₂Fe ₄Se ₅ powder and superconductors in the K _xFe _2-ySe ₂ system, and examined differences in their structures by high-resolution synchrotron and single-crystal x-ray diffraction. We found four distinct phases: semiconducting K ₂Fe ₄Se ₅, a metallic superconducting phase K _xFe ₂Se ₂ with x ranging from 0.38 to 0.58, the phase KFe _1.6Se ₂ with full K occupancy and no Fe vacancy ordering, and a oxidized phase K _0.51(5)Fe _0.70(2)Se that forms the PbClF structure upon exposure to moisture. We find that the vacancy-ordered phase K ₂Fe ₄Se ₅ does not become superconducting by doping, but the distinct iron-rich minority phase K _xFe ₂Se ₂ precipitates from single crystals upon cooling from above the vacancy ordering temperature. This coexistence of separate metallic and semiconducting phases explains a broad maximum in resistivity around 100 K. Further studies to understand the solubility of excess Fe in the K _xFe _2-ySe ₂ structure will shed light on the maximum fraction of superconducting K _xFe ₂Se ₂ that can be obtained by solid state synthesis.