Cu/Te substitution effects on superconductivity and microstructure of phase-separated K_0.8Fe_1.75Se₂

Two series of K_0.8Fe_1.75-xCu_xSe ₂ (0 ≤ x ≤ 0.15) and K_0.8Fe_1.75Se _2-yTe_y (0 ≤ y ≤ 0.5) single crystals of nominal composition have been prepared and their physical properties and microstructural features have been studied. Resistivity measurements demonstrate that the superconducting transition temperature decreases gradually with the increase of the substitution level and zero resistivity finally disappears in both systems. Systematic TEM, SEM and XRD structural analyses, in combination with the magnetic experimental data, reveal a rich variety of structural phenomena resulting from different types of substitution. Cu substitution gives rise to the volume of a new non-superconducting Cu-rich phase with modulation q ₃ = 1/3[2a*+ b*] coexisting with the q₂ = [a*+ b*] superconducting stripe domain. With the increase of the ratio of the new non-superconducting phase along with doping, the superconducting path is finally cut off, and results in the absence of zero resistivity. In contrast, the absence of superconductivity in Te-substituted materials is correlated with the complete disappearance of the q₂ superconducting phase due to the suppression of phase separation.