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.
|Original language||English (US)|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - Nov 19 2012|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics