Structures of Fe3O4 (111) surfaces observed by scanning tunneling microscopy

Scanning tunneling microscopy (STM) has been used to image the (111) surface of magnetite (Fe3O4), following argon-ion bombardment and annealing in O-2. The (1 x 1) hexagonal low-energy electron-diffraction (LEED) pattern obtained from this surface is consistent with a bulk-terminated Fe3O4 (111) surface, STM images show two distinct coexisting surface terminations having identical hexagonal unit-cell dimensions and orientations. We designate these terminations A and B. Termination A shows close-packed features separated by 6.1+/-0.2 Angstrom, while termination B shows features separated by 3.6+/-0.4 Angstrom which are arranged in a honeycomb pattern. The step height from termination A down to termination B is 3.8+/-0.5 Angstrom, while that from termination B down to termination A is 0.5+/-0.2 Angstrom. A model is proposed which identifies these terminations with the two types of Fe(lll) layers found in the bulk Fe3O4 structure. Termination A corresponds to an unreconstructed (111) termination of Fe3O4 that exposes 3/4 ML of Fe atoms and 1/4 ML of O atoms over a close-packed O layer, with each feature arising from a trimer of Fe atoms capped by an O atom. Termination B corresponds to an unreconstructed (111) termination that exposes 1/2 ML of Fe atoms over a close-packed O layer; each feature is due to a single Fe atom. Ionic and covalent models of surface stability indicate that the atomic arrangement proposed for termination B would be more stable than that for termination A.