Combined density functional theory and interatomic potential study of the bulk and surface structures and properties of the iron sulfide mackinawite (FeS)

The iron sulfide mackinawite (FeS) is modeled by density functional theory calculations, the results of which are used to derive a set of interatomic potentials for this material. We have investigated the effect of adding a Hubbard U-eff term to the gradient-corrected (GGA) functional, but whereas good agreement is shown with experimental data when pure GGA (U-eff = 0) is employed, for values of U-eff greater than zero the solution becomes unphysical, an indication that the Fe valence electrons within mackinawite are delocalized. This property is further evidenced by the density of states, which confirms a metallic nature from delocalization of the Fe d orbital at all U-eff values. Mackinawite is calculated to be a nonmagnetic material, in accord with experiment. We have also derived a set of interatomic potentials by fitting to observables. including geometry, phonon frequencies, and elastic constants, which were calculated for isolated mackinawite layers using DFT. The derived potential model is used to calculate the unrelaxed and relaxed structures and energies for the {100}, {010}, {001}, {110}, {101}, {011}, and {111} surfaces of mackinawite. The {001} surface, tenninating at S atoms, with a calculated surface energy of 0.07 J m(-2), is found to be the most stable surface with very little surface relaxation compared to the bulk material.