Density functional study of the optical response of FeN and CoN nitrides with zinc-blend and rock-salt structures

We investigated the optical properties of iron and cobalt mono-nitrides in rock-salt and zinc-blende crystal structures. Density functional theoretic calculations were performed using the linear muffin tin orbital method in the generalized gradient approximation for exchange and correlation. The optical response was characterized by means of the dielectric function, calculated for each structural phase in the different magnetic arrangements found experimentally and theoretically. The origin of the main absorption peaks was traced back to particular interband transitions after a careful analysis of the band structures and orbital- and atom- projected electronic densities of states. The optical response of both nitrides with this structure in the nomagnetic state is very similar at photon frequencies below 2 eV, but differ at high frequencies. In zinc-blende structure, both compounds are nomagnetic and present an optical gap of about 0.5 eV for interband transitions in their optical response.