Density-functional theory studies of pyrite FeS2(1 0 0) and (1 1 0) surfaces

We have performed density-functional theory calculations using both plane wave-pseudopotential and Gaussian basis set approaches on the (1 0 0), planar (1 1 0) and microfacetted (1 1 0) surfaces of pyrite (FeS2). Our calculations indicate that the (1 0 0) surface is more stable than the planar (1 1 0) surface, which is predicted to have a higher surface energy. Creation of microfacets on the (1 1 0) surface resulted in a lower surface energy. Relatively small differences in calculated surface energy between the ideal and relaxed (1 0 0) and (1 1 0) surfaces were found. The (1 0 0) and (1 1 0) surfaces are predicted to be essentially bulk-terminated, with a relatively small amount of relaxation. Electrostatic effects induced by loss of coordination at surface Fe atoms are likely to be responsible for surface ionic displacements. Our calculations indicate that surface Fe atoms of fourfold coordination, present on the (1 1 0) surfaces, are spin polarized, while those of fivefold coordination are fully spin-paired. These results suggest that magnetic species, such as O2, are more prone to react at low Fe coordination defect sites on real FeS2 (1 0 0) surfaces.