Density-functional theory studies of pyrite FeS2 (1 1 1) and (2 1 0) surfaces

We have performed DFT calculations using both plane wave-pseudopotential and Gaussian basis set approaches on the (1 1 1) and (2 1 0) surfaces of pyrite (FeS2). Our calculations indicate that the (1 1 1) surface is more stable than the (2 1 0) surface, which is predicted to have a higher surface energy. The (2 1 0) surface is predicted to be essentially bulk terminated, with a relatively small amount of surface relaxation. Bridging S atoms on the (1 1 1) surface undergo significant lateral displacement to coordinate to neighbouring undercoordinated surface Fe atoms. 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 states dominate at the Fermi level on the (2 1 0) surface, but that surface S states make significant contributions to the valence band on the (1 1 1) surface. On the (2 1 0) surface, Fe atoms of 4-fold coordination are spin polarized and therefore paramagnetic, while 5-fold coordinated Fe are diamagnetic. For the (1 1 1) surface, both 5- and 6-fold coordinated Fe atoms are predicted to be spin polarized, although nominally the unpaired electrons are expected to be localised on the surface S− species. This is likely to be due to the removal of spin polarization from the surface S by charge transfer from the surface Fe.