Growth Kinetics, Cation Occupancy, and Magnetic Properties of Multimetal Oxide Nanoparticles: A Case Study on Spinel NiFe2O4

Growth kinetics of multimetal oxide nanomaterials, the key for structural and property tailoring, is extremely difficult to be determined due to the crystal cracking and the quick migration of metal ions in lattice originated from the big strains after long-term calcinations at very high temperature. Herein, spinel NiFe2O4 was taken as a model multimetal oxide for a case study. NiFe2O4 nanoparticles were synthesized to show grain sizes ranging from 4.3 to 780 nm by a sol gel autocombustion method. Through a systematic sample characterization via XRD, SEM, and TEM, the grain growth mechanism for NiFe2O4 nanoparticles is uncovered to follow an Ostwald ripening model and, grain boundary diffusion model, totally different from those mechanism previously reported for simple oxide nanoparticles, such as LaMer mechanism, Ostwald ripening, oriented attachment mechanism, and their combination. With this unique model, the grain growth kinetics of NiFe2O4 nanoparticles below 700 degrees C was determined to quantitatively show an equation, D-5 = (1.37 x 10(15))t exp(-150.75/RT). With the grain growth ongoing, Fe3+ ions of surface/interface might migrate to the octahedral site of NiFe2O4 lattice in the Ostwald ripening process, while more Fe3+ ions at the surface/interface of NiFe2O4 nanoparticles would occupy the tetrahedral sites during the grain -boundary diffusion process. Further, the cell volume showed a minimum at D = 120.9 nm, which was interpreted in terms of the balance between strong repulsive interaction of the parallel defect dipoles at surfaces and the lattice strain due to the migration of Ni2+ and Fe3+ from interface to the bulk. Eventually, as particle size increases, the room-temperature saturation magnetization, M-s, increased continuously, while other magnetic parameters (e.g., coercivity H-c, remanent M-r, and paramagnetic susceptibility chi(p)) showed crossovers. These observations were interpreted in terms of the cation occupancy and lattice variations. The findings reported in this work provide an unprecedented understanding of growth kinetics of spinel NiFe2O4 NPs and may pave an elegant route to synthesize multimetal oxide nanomaterials with specific structure and magnetic properties.