Direct observation of nucleation embryos during NiFe2O4 precipitation in NiO

Very small NiFe2O4 particles between 1 and 4 nm in size have been observed in an Fe-doped NiO sample quenched during early stages of spinel precipitation. The differences between the crystal structure of NiFe2O4 and NiO made it possible to use three different TEM imaging techniques to observe these very small particles. These very small particles are distributed throughout the volume of the sample and hence were not produced by specimen preparation. The large concentration of the very small precipitates compared with the much larger but less numerous growing precipitates indicates that the very small precipitates are actually embryos which form due to compositional fluctuations. The critical size for spinel nucleation was estimated to be 4.3 nm based on the distribution in the size of very small particles. The embryos are bounded by (111) planes with an octahedral morphology which is the minimum energy morphology. © 1992, Materials Research Society. All rights reserved.