STRUCTURES OF FE3O4(111) SURFACES OBSERVED BY SCANNING-TUNNELING-MICROSCOPY

Scanning tunneling microscopy (STM) has been used to image the (111) s urface of magnetite (Fe3O4), following argon-ion bombardment and annea ling in O-2. The (1 x 1) hexagonal low-energy electron-diffraction (LE ED) pattern obtained from this surface is consistent with a bulk-termi nated Fe3O4 (111) surface, STM images show two distinct coexisting sur face terminations having identical hexagonal unit-cell dimensions and orientations. We designate these terminations A and B. Termination A s hows close-packed features separated by 6.1+/-0.2 Angstrom, while term ination B shows features separated by 3.6+/-0.4 Angstrom which are arr anged 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 fo und in the bulk Fe3O4 structure. Termination A corresponds to an unrec onstructed (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 a rising from a trimer of Fe atoms capped by an O atom. Termination B co rresponds 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 sing le Fe atom. Ionic and covalent models of surface stability indicate th at the atomic arrangement proposed for termination B would be more sta ble than that for termination A.