  Superconductors and multiferroics are two of the hottest branches in
condensed matter physics. The connections between those two fields are
fundamentally meaningful to unify the physical rules of correlated electrons.
Recently, BaFe$_2$Se$_3$, was predicted to be multiferroic [Phys. Rev. Lett.
113, 187204 (2014)] due to its unique one-dimensional block-type
antiferromagnetism. Here, another iron-selenide KFe$_2$Se$_2$, a parent state
of iron-based superconductor, is predicted to be multiferroic. Its
two-dimensional block-type antiferromagnetism can generate a moderate electric
dipole for each Fe-Se layer via the Fe-Se-Fe exchange striction. Different
stacking configurations of these magnetic blocks give closely proximate
energies and thus the ground state of KFe$_2$Se$_2$ may be switchable between
antiferroelectric and ferroelectric phases.
