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 O₂. The (1×1) 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 Å, while termination B shows features separated by 3.6±0.4 Å which are arranged in a honeycomb pattern. The step height from termination A down to termination B is 3.8±0.5 Å, while that from termination B down to termination A is 0.5±0.2 Å. A model is proposed which identifies these terminations with the two types of Fe(111) layers found in the bulk Fe₃O₄ structure. Termination A corresponds to an unreconstructed (111) termination of Fe₃O₄ that exposes 34 ML of Fe atoms and ¼ 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 ½ 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.