Synthesis, characterization, and structural determination of ferrispinels

Abstract

Spinel (AB2X4) crystalline system is typically characterized by a distribution of cations that can move from tetrahedral (A) to octahedral (B) sites. This movement also involves the X anions rearrangement into the cubic lattice. Accordingly, spinels can exhibit A-sites and B-sites distribution characterized by a partial to total cell inversion degree (x=1). We are interested in Zn-ferrites due to their chemical and thermal stability, ferrimagnetic properties, and earth abundance. ZnFe2O4 shows the unitary cell for a typical spinel structure; however, some previous reports suggested that this spinel can be inverted at the nanoscale regime. Hence, it is expected that ZnFe2O4 nanoparticles might show a ferrimagnetic behavior, which is an outstanding property for several applications, including high-density magnetic data storage and water splitting. The structural determination and microstructure properties are of great value in analyzing the cell inversion degree for particles synthesized following a proposed experimental design.

In this Thesis, we synthesized nano and sub-micro, beam-like, and amorphous particles using a hydrothermal method varying the proportion of reactants, reaction time, and reagents. The morphology and size of the as-prepared particles were characterized by scanning electron microscopy. Atomic chemical composition and elemental stoichiometry were determined using energy-dispersive X-ray and inductively coupled plasma optical emission spectroscopy. Raman spectroscopy measurements indicate redshifts notably observed for the symmetric mode. Such wavenumber increments suggest that the frequency of phonons interacting with the incident photon is decreasing, probably due to the improved crystallinity during annealing treatments. The crystalline evolution was followed by X-ray diffraction. At the same time, lattice parameters were obtained from Rietveld refinements, and microstructure properties were assessed via Williamson-Hall type fittings. The cell parameter is estimated to be about 8.44 nm. The crystallite sizes range from 9 to 65 nm, and the microstrain is less than 0.2%. Finally, the degree of inversion of the crystalline system was evaluated using the Bertaut method. The cell inversion degree follows as 0.85 (400 °C), 0.67 (600 °C), and 0.38 (800 °C), suggesting that the annealing process helps to restore a standard spinel structure. Our simple synthesis method facilitates the tuning of the size and shape of particles, which appropriately leads to improved crystallinity as observed from structural parameters and cell inversion degrees. In this way, we assisted in evaluating and comparing the synthesis parameters to obtain ZnFe2O4 particles with different cell inversion degrees, sizes, and microstrain. We believe that this analysis could be replicated in other spinel structures and might help evaluate the relationship between their structure and magnetic properties.