AN IN-SITU STUDY OF THE SURFACE PHASE-TRANSITIONS OF ALPHA-FE2O3 BY X-RAY-ABSORPTION SPECTROSCOPY AT THE OXYGEN K-EDGE

Surface modifications of single crystal alpha-Fe2O3 have been monitore d as a function of different in situ heat treatments after ion etching , in UHV and under controlled pO(2). The particular (T, pO(2)) conditi ons were chosen to correspond to the stability domain of Fe3O4 and Fe2 O3 in the bulk Fe-O phase diagram. X-ray absorption spectroscopy was p erformed, in-situ, at the oxygen K edge, enabling the identification o f the interconversion, Fe2O3 <-> Fe3O4-x, in the surface layers. Where as re-oxidation of the surface under UHV is by thermal diffusion of ox ygen from the bulk, the evidence suggests that re-oxidation under 10(- 6) mbar of oxygen involves a more complex mechanism.