Abstract
We report the effect of a polyol-mediated annealing on nickel ferrite nanoparticles. By combining X-ray fluorescence spectroscopy, X-ray diffraction, and 57Fe Mössbauer spectrometry, we showed that whereas the as-prepared nanoparticles (NFO) are stoichiometric, the annealed ones (a-NFO) are not, since Ni0-based crystals precipitate. Nickel depletion from the spinel lattice and reduction in the polyol solvent are accompanied with an important cation migration. Indeed, thanks to Mössbauer hyperfine structure analysis, we evidenced that the cation distribution in NFO departs from the thermodynamically stable inverse spinel structure with a concentration of tetrahedrally coordinated Ni2+ of 20 wt-% (A sites). After annealing, and nickel demixing, originated very probably from the A sites of NFO lattice, the spinel phase accommodates with cation and anion vacancies, leading to the (Fe3+0.84□0.16)A[Ni2+0.80Fe3+1.16□0.04]BO4-0.20 formula, meaning that the applied polyol-mediated treatment is not so trivial.
Highlights
IntroductionThey had attracted much interest since Néel’s discovery of ferrimagnetism and antiferromagnetism in 1948 (Néel, 1948)
Spinel ferrites are an old class of functional ceramic materials
The X-ray diffraction (XRD) pattern recorded on fresh NFO powder (Figure 1) corresponds very well to that of the nickel ferrite cubic spinel structure (ICDD n°98-002-8108), and that of the annealed one does not (Figure 1)
Summary
They had attracted much interest since Néel’s discovery of ferrimagnetism and antiferromagnetism in 1948 (Néel, 1948) Due to their high resistivity and the tailorability of their magnetic properties as a function of their chemical composition and local structure, they have been considered for a large number of electromagnetic applications: magnetic sensors, permanent magnets, electronic inductors, transformers, magnetic recording tapes, and radar-absorbing coatings (Goldman, 2006). The structure of these oxides derives from that of spinel magnetite, with formula Fe3O4, which consists of a face-centered cubic (fcc) oxygen lattice where the iron cations occupy the tetrahedral (A) and octahedral (B) interstitial sites as follows: (Fe3+)A[Fe2+Fe3+]BO4. This structure is called the inverse spinel structure by opposition to the direct structure, in which the divalent and the trivalent cations are exclusively located in the A and the B sites, respectively (Smith and Wijn, 1961)
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