OXYGEN-ADSORPTION ON MON PRECOVERED FE-3.S-PERCENT-MO-N(100) SINGLE-CRYSTALS

The two-dimensional MoN surface compound has been prepared on the Fe-3 .5%Mo-N(100) surface by means of surface cosegregation of molybdenum a nd nitrogen. The MoN precovered Fe-3.5%Mo-N(100) surface shows a sharp (1 x 1) low energy electron diffraction (LEED) pattern. The structure of the MoN surface compound has been determined by X-ray photoelectro n diffraction to consist of a single MoN surface layer plus an additio nal Mo subsurface layer. AES and LEED have been utilized to study the dissociative oxygen adsorption on the MoN precovered Fe-3.5%Mo-N(100) surface. The experiments have been conducted at oxygen pressures rangi ng from 5 x 10(-9) to 1 x 10(-6) mbar and temperatures between 400 and 550 degrees C. It has been found that chemisorbed oxygen atoms replac e nitrogen atoms of the MoN surface compound and cause a reduction of the Mo surface concentration. After similar to 1001 O-2 the initial ex change process is largely completed and leads to an oxygen and molybde num rich (MoxFe1-x)(OyN1-y) surface phase. The (MoxFe1-x)(OyN1-y) cove red substrate shows a sharp (1 x 1) LEED pattern indicating epitaxial stabilization of the (MoxFe1-x)(OyN1-y) surface phase on the Fe-3.5%Mo (100) surface. It is shown that the structure of the (MoxFe1-x)(OyN1-y ) surface phase is related to the (100) plane of the rocksalt structur e. Further oxidation causes the formation of inhomogeneous iron oxide layers on MoN precovered and sputter cleared Fe-3.5%Mo-N(100) surfaces as well. (C) 1997 Elsevier Science B.V.