Fe-site substitution effects on superconductivity and microstructure of phase-separated K_0.8Fe_1.75Se₂ superconductor

Series of Fe-site-substituted K_0.8Fe_1.75-xM _xSe2 (M = Cu, Ni, Mn, Zn, Co, Cr) single crystals of nominal composition have been prepared and microstructural features have been studied. Systematic Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) investigations reveal that the substitution effect can be classified into two types: Ni, Co and Cr atoms can be homogenously doped into the superconducting stripes of the phase-separated K_0.8Fe _1.75Se₂ system, while the Cu, Mn and Zn atoms can hardly be introduced into the superconducting stripes, which instead give rise to the formation of new non-superconducting K-M-Se phases embedded in the sample. The complex physical measurement data reported in the literature can be nicely explained by the microstructure picture.