  In this paper we study an effective model for the normal state of iron-based
superconductors. It has separate, but interacting itinerant and localized
degrees of freedom, originating from the d_xz and d_yz, and from d_xy iron
orbitals respectively. At low temperatures, below a mean-field phase
transition, these different states condense together in an excitonic order
parameter. We show that at even lower temperature, after another phase
transition, this ordered state can spontaneously break the C_4 lattice symmetry
and become nematic. We propose this mechanism as an explanation of the tendency
towards nematicity observed in several iron-based compounds.
