Effects of Thermal Annealing on Structural and Magnetic Properties of Lithium Ferrite Nanoparticles

Pure, crystalline, similar to 10 nm lithium ferrite phase (Li0.5Fe2.5O4), was Successfully synthesized at very low temperature using a modified combustion method. The crystal structure and microstructure evolution of this system upon annealing were monitored by a careful investigation of X-ray diffractograms collected on a synchrotron source. Comparative analysis of the results obtained from the full profile Rietveld method (in reciprocal space) and the pair distribution function method (in direct space) was carried Out. Nanocrystalline samples exhibit similar crystal structure, on average, with a partial ordering of Li+ and Fe3+ ions between octahedral 4b and 12d sites on the spinel crystal lattice (space group P4(3)32). After annealing at 973 K, cation distribution changes to a completely ordered, resembling that which is seen in the bulk lithium ferrite. The PDF analysis reveals abnormally high values of oxygen atomic displacement parameters in tetrahedral 8c sites (OI) indicating a significant disordering of the OI network and suggests migration of lithium ions from 4b sites to the outer layers of nanoparticles. Analysis of room temperature Mossbauer spectra has shown that the hyperfine field for Fe3+ ions in tetrahedral 8c sites is the most sensitive on increasing the particle size and improving the crystallinity. From the differential thermal analysis, it was found that a lower driving force is required to induce an order-disorder phase transition in nanocrysalline samples, compared to the bulk-like sample, presumably due to the higher crystal disordering in these samples.