Iron nitrides are of great interest due to their mechanical, chemical and magnetic properties. In the work presented here, electronic structure calculations have been performed in order to evaluate magnetic moments for different related structures. The underlying idea was to introduce small amounts of substitutional atoms in order to produce a lattice expansion likely to enhance magnetic properties. Results concerning substitution of iron by gold are presented and discussed. Calculations reported here show the role of lattice expansion on magnetic properties and open the way of a priori determination of optimal chemical composition for enhanced magnetic behaviour.