Fe3O4 (111) formation on a reduced α-Fe2O3 (11(2)over-bar3) substrate:: a low-energy electron diffraction and scanning tunnelling microscopy study

A (11 (2) over bar 3) surface of alpha-Fe3O3 (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 (1123) plane. Images obtained by scanning tunnelling microscopy (STM) from the prepared alpha-Fe2O3(11 (2) over bar 3) surface show terraces of close-packed features. These features are separated by 6.0+/-0.5 Angstrom and, together with step heights of 4.8+/-0.5 Angstrom which separate these terraces, are consistent with previously reported STM images of Fe3O4(111) surfaces. Features ;corresponding to a bulk termination of alpha-Fe2O3(11 (2) over bar 3) were not observed. We propose that Fe3O4(111) nucleates on the reduced alpha-Fe2O3(11 (2) over bar 3) substrate, with the [1 (1) over bar 00] direction of alpha-Fe2O3 parallel to the [(1) over bar 10] direction of Fe3O4. This epitaxial relationship is favoured by substrate oxygen planes parallel to alpha-Fe2O3(11 (2) over bar 3), and by close-packed oxygen planes parallel to Fe3O4(111). (C) 2000 Elsevier Science B.V. All rights reserved.