Abstract
A stepwise mechanism is outlined for diffusion-controlled oxidation of a metal Me to a metal deficit oxide Me3−zO, where z is the normal valence of Me in Me3−zO and may equal either 1 or 2. Specifically, oxidation is postulated to occur by (1) chemisorption of a singly ionized oxygen atom O− on the oxide surface with the concomitant formation of an electron holeprobably localized on a lattice site as a more electropositive cation Mez+1, (2) subsequent further ionization of chemisorbed O− and its incorporation into the oxide with the formation of singly charged cation vacancies V′ and, in the case z=1, an additional compensating hole Mez+1, (3) migration of Mez ions and electrons via the V′ and Mez+1 defects from the underlying metal to the surface, and (4) annihilation of the defects V and Mez+1 at the oxide-metal interface by the passage of metal atoms from the metal into the oxide. Such a mechanism leads to an eighth root pressure dependency for monovalent cation systems like O2-Cu2O-Cu and a fourth root pressure dependency for bivalent systems like O2-NiO-Ni and O2-CoO-Co. The behavior of copper and nickel are shown to be as predicted; whereas, in the case of cobalt, defect interaction results in deviations from the predicted behavior.
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