  We study antimony doping effects in the iron-based superconductor
CaFe(Sb$_{x}$As$_{1-x}$)$_{2}$ by using the first-principle calculation. The
calculations reveal that the substitution of the doped antimony atom into As of
the chainlike As layers is more stable than that in FeAs layers. This
prediction can be checked by experiments. Our results suggest that doping
homologous elements into the chainlike As layers existing only in novel 112
system is responsible for rising up the critical temperature. We discuss
antimony doping effects on the electronic structure. It is found that the
calculated band structures with and without the antimony doping are similar to
each other within our framework.
