Oxygen segregation on iron surfaces in equilibrium with Fe-FeO at 800°C

Abstract

The oxygen adsorption was studied on Fe(100), Fe(110) and Fe(111) surfaces after establishing the segregation equilibrium O(dissolved in iron) → O(adsorbed) at 800°C for the oxygen activity given by the equilibrium of Fe and FeO. To achieve this equilibrium the iron single crystals were sintered together with a tablet of an Fe-FeO mixture. The oxygen concentration on the single crystal surface was measured after equilibration at 800°C by AES, and the LEED patterns were observed at 800°C and after quenching to room temperature. On Fe(100) the adsorption structure p(1 × 1) was present at 800°C and after quenching to room temperature. Assuming that the adsorption structure is saturated, a calibration is obtained for the AES measurements. On Fe(110) at 800°C the degree of coverage is relatively low and no adsorption structure could be detected. Most probably the oxygen atoms are mobile at 800°C on Fe(110). Upon quenching, the oxygen atoms cluster together under formation of oxygen layers ordered similarly as in FeO(111) planes. The Fe(111) surface is not stable at 800°C in equilibrium with the Fe-FeO mixture, FeO crystallites begin to grow in the epitaxial relation Fe(111) // FeO(111). Such experiments with couples: metal single crystal/metal-metal oxide mixture, give unequivocal information on the adsorption states and surface phenomena just at and below the equilibrium pressure for oxide formation.