A LEES-AES [i.e. LEED-AES] study of the initial oxidation of the cobalt (0001) surface

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1978

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The initial oxidation of the cobalt (0001) surface was studied by LEED and AES. Since cobalt undergoes a phase transforation (hcp -»fcc) at about 420°C, special care was taken in the heat treatment and cleaning of the sample surface. First, a clean (0001) surface was studied by LEED above and below the phase transformation and the abrupt change in LEED pattern at the transformation temperature was observed.- Oxidation was done at various temperatures between room temperature and 740°C. In the room temperature oxidation, a clear chemisorption stage was observed by both the oxygen uptake curve and LEED fractional spots. It took high oxygen exposure to obtain an oxide LEED pattern due to oxygen. diffusion into the bulk. At high temperatures, the chemisorption stage was formed by the same oxygen exposure as in the case of room temperature, but the oxygen concentration at the surface did not increase sufficiently to give an oxide LEED pattern. High diffusivity of oxygen in cobalt was observed by taking the depth profile of an oxidized surface and by a hydrogen reduction experiment, The model of the oxide formation is presented based upon the high oxygen diffusivity, The difference in the diffusivity between the two phases (hep and fee) was not detected, The high oxygen diffusivity and the resulting slow oxide formation are attributed to the defects formed in the bulk by the heat treatment through the phase transformation.

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