First-principles study of the preference for zinc-blende or rocksalt structures in FeN and CoN

The energies of zinc-blende and rocksalt structures FeN and CoN were calculated as function of lattice constant using the local density approximation and generalized gradient approximation density functional methods and the linear muffin-tin orbital band-structure method. The zinc-blende structure is found to be preferred in both cases at ambient pressure. The transition pressure for the zinc-blende to rocksalt phase transition is calculated to be about 50 and 30 GPa for FeN and CoN, respectively. Spin-polarized calculations indicate that rocksalt FeN has a small but nonzero magnetic moment near its equilibrium lattice constant, while CoN has zero magnetic moment below a certain critical value of the lattice constant. The magnetic moment increases abruptly near a certain critical lattice constant for both materials. The zinc-blende phases are found to be nonmagnetic. In FeN a ferromagnetic ordering is predicted but the energy difference with the antiferromagnetic AFM-I alignment is very small. Densities of states and band structures are given for both structures and used to discuss the structural preference and magnetic behavior.