  Recent experiments on Zn-substituted 122-type iron-based superconductors
(FeSCs) at electron- and hole- doped region provide us with a testing ground
for understanding the effect of Zn impurities in these systems. Our
first-principle calculations of the electronic structure reveal that the Zn 3d
orbitals are far below the Fermi level and chemically inactive, while the Zn
4s-orbital is partially occupied and its wave function overlapping with those
3d-orbitals of neighboring Fe-ions. This suggests that the impurity effect is
originating in the Zn 4s-orbital, not its 3d-orbitals. Employing a
phenomenological two-orbital lattice model for 122-FeSCs and the
self-consistent Bogoliubov-de Gennes equations, we study how the Zn-impurities
suppress the superconductivity in electron- and hole- doped compounds. Our
obtained results qualitatively agree with the experimental measurements.
