Theoretical study of the termination of the Fe3O4 (111) surface

Ab-initio periodic Hartree-Fock calculations for the structure of the (111) surface of Fe3O4 (magnetite) are presented. The slabs that are derived by an ideal bulk truncation that leaves one or two iron monolayers outside oxygen layers are found to be more stable than others since they preserve most of the coordination of the surface atoms. The stability of the slabs that represent the surface layers depends on the overall composition, specifically on the deviation from stoichiometry, and on the dipole moment perpendicular to the surface. The symmetrical slab with the layer sequence Fe2O4Fe3O4Fe2, terminated on each side by iron bilayers, is the best compromise since symmetry insures the neutrality of the dipole moment. This slab is oxygen-deficient. The energetically preferred structure relaxes so that one of the two outermost iron layers moves toward the slab center plane, exchanging sequence with the oxygen layer. The slab with the layer sequence FeO4Fe3O4Fe, which is also symmetric, is terminated by iron single monolayers, would represent an excessive oxidation of the iron atoms. This slab may be reduced by hydrogenation; it is then strongly stabilized and the vertical displacement of the oxygen atoms agrees with the structure determined by LEED (the LEED study would not have detected hydrogen). (C) 1999 Elsevier Science B.V. All rights reserved.