Structural steps to oxidation of Ni(100)

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Abstract
In this paper, we emphasize the temperature- and exposure-dependent development of low-energy electron diffraction patterns,measured quantitatively during oxidation of Ni(100) at 80 to 400 K. We find a strong temperature dependence in the development of LEED patterns associated with NiO. NiO(111) is favored by adsorption temperatures below 300 K, whereas a (7×7)-like structure is favored by adsorption temperatures of 300 to 400 K. Room temperature is a ‘‘crossover’’ point between these two forms of the oxide. The final oxide depth is independent of adsorption temperature and, therefore, of epitaxial orientation, between 80 and 400 K. When the sample is heated in vacuum after adsorption, massive rearrangements take place above 500 K. Some of the nickel reverts to metallic nickel covered by a c(2×2) oxygen overlayer, and some forms NiO crystallites which are probably deeper than the initial oxide skin. Effectively, the parent oxide disproportionates into a less-oxygen-rich phase and a more-oxygen-rich phase. This is again independent of the orientation of the initial oxide epitaxy.