The cation tracer diffusion coefficient
D and the electric conductivity σ in NiO were measured as a function of temperature and oxygen partial pressure
PO2. Polycrystalline NiO sample was prepared by oxidation of zone refined nickel platelets in an oxygen stream. For conductivity measurement single crystal was also used. It was found that σ was proportional to
PO21⁄4∼1⁄5, regardless of the method of sample preparation. This suggests strongly the superiority in concentration of the singly ionized vacancy
V′. On the other hand,
D depended very slightly on
PO2, much less than the 1/4 power. This discrepancy of the
PO2 dependence of σ and
D can be successfully explained by the following assumption: The cation diffusion takes place in a dual manner, through singly ionized vacancies
V′ and through doubly ionized vacancies
V″. The contribution of
V′ to the diffusion is predominant in the higher
PO2 region because of its high concentration, whereas that of
V″ is more important in the lower
PO3 region because of its large mobility.
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