MORPHOLOGY OF NIFE2O4 PRECIPITATION IN NIO

The morphology of NiFe2O4 precipitates in a NiO matrix has been studied using TEM. The spinel precipitates had a dendritic morphology when a polycrystalline sample of Fe-doped NiO was cooled at a constant rate from above the solvus temperature. They nucleate as octahedra bounded by {111} interfaces which then grow by a ledge mechanism until they reach a critical size and this shape becomes unstable. The precipitates then adopt a dendritic morphology characterized by branches in the <001> directions. The branches are bounded by {111} and {011} facets. HREM showed that during the initial stages of growth the interface is composed of a series of closely spaced ledges which are typically one spinel lattice-spacing high. Later, the ledge spacing decreases resulting in large flat {111} and {011} facets. Secondary branches were observed to form under large supersaturations of Fe when the precipitate spacing was large. The shape of the precipitates during the early stages of dendritic growth are very similar to those predicted by a Monte Carlo simulation which incorporates diffusional growth and anisotropic interface kinetics. The dendritic morphology is associated with the small lattice misfit, the large diffusion rate in Fe-doped NiO and the large undercooling necessary for nucleation. The diffusion rate is particularly large due to the concentration of cation vacancies being approximately half of the iron concentration.