Phase separation in the iron chalcogenide superconductor Fe _1+yTe_xSe_1-x

We present direct evidence for phase separation and chemical inhomogeneity in Fe_1+yTe_xSe_1-x single crystals from scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) data. In STEM, images recorded using a high-angle annular dark field (HAADF) detector show characteristic nanometer-scale patterns of phase separation from the Z-dependent image contrast. The separation was observed both in nonsuperconducting samples with excess iron and in superconducting samples. Using EELS, we determined ̃20% (or lower) fluctuation in Te concentration from the average local compositions by integrating the intensity of the Te-M_4,5 edge. The energy-loss near-edge structure (ELNES) of the Fe-L_2,3 edge changes with varying composition, especially the L ₃/L₂ white-line intensity ratio, which is sensitive to the d-state occupancy of the Fe. The results suggest a miscibility gap in the Fe_1+yTe_xSe_1-x system and changes in the d-electron states at the nanometer scale from the separated phases.