Bulk and surface phases of iron oxides in an oxygen and water atmosphere at low pressure

Thermodynamic stability ranges of different iron oxides were calculated as a function of the ambient oxygen or water gas phase pressure (p less than or equal to1 bar) and temperature by use of the computer program EquiTherm. The phase diagram for Fe-H2O is almost completely determined by the O-2 pressure due to the H2O dissociation equilibrium. The formation of epitaxially grown iron oxide films on platinum and ruthenium substrates agrees very well with the calculated phase diagrams. Thin films exhibit the advantage over single crystals that bulk diffusion has only limited influence on the establishment of equilibrium phases. Near the phase boundary Fe3O4-Fe2O3, surface structures are observed consisting of biphase ordered domains of FeO(111) on both oxides. They are formed due to kinetic effects in the course of the oxidation to hematite or reduction to magnetite, respectively. Annealing a Fe3O4(111) film in 5x10(-5) mbar oxygen at 920-1000 K results in a new gamma -Fe2O3(111)-like intermediate surface phase during the oxidation to alpha -Fe2O3(0001). A model is suggested for the growth of iron oxides and for redox processes involving iron oxides. The formation of several equilibrium surface phases is discussed.