Cation Transport Phenomena in NiO

Abstract

The cation tracer diffusion coefficient D and the electric conductivity σ in NiO were measured as a function of temperature and oxygen partial pressure P_O2. 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 P_O2^{1⁄4∼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 P_O2, much less than the 1/4 power. This discrepancy of the P_O2 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 P_O2 region because of its high concentration, whereas that of V″ is more important in the lower P_O3 region because of its large mobility.