Fe₃O₄(111) formation on a reduced α-Fe₂O₃(112̄3) substrate: A low-energy electron diffraction and scanning tunnelling microscopy study

A (112̄3) surface of α-Fe₂O₃ (haematite) was prepared by Ar⁺-ion sputtering and annealing in ultrahigh vacuum at 1123 K. Examination of this surface by low-energy electron diffraction (LEED) reveals a complex diffraction pattern from which we identify two distinct reciprocal unit cells. The first is identical to that observed for a (111) bulk termination of Fe₃O₄ (magnetite); the second corresponds to a bulk termination of the (112̄3) plane. Images obtained by scanning tunnelling microscopy (STM) from the prepared α-Fe₂O₃(112̄3) surface show terraces of close-packed features. These features are separated by 6.0±0.5 angstroms and, together with step heights of 4.8±0.5 angstroms which separate these terraces, are consistent with previously reported STM images of Fe₃O₄(111) surfaces. Features corresponding to a bulk termination of α-Fe₂O₃(112̄3) were not observed. We propose that Fe₃O₄(111) nucleates on the reduced α-Fe₂O₃(112̄3) substrate, with the [11̄00] direction of α-Fe₂O₃ parallel to the [1̄10] direction of Fe₃O₄. This epitaxial relationship is favoured by substrate oxygen planes parallel to α-Fe₂O₃(112̄33), and by close-packed oxygen planes parallel to Fe₃O₄(111).