A powder neutron diffraction investigation of vacancy ordering and covalence in γ-Fe2O3

Pure hematite nanoparticles (HNPs) were successfully synthesized via the simple precipitation route by using ferric sulfate precursor and ammonium hydroxide (NH4OH) precipitant. The current study reports the effects of precursor concentrations, from 0.05 to 0.45 M, on the product size, morphology, crystallinity, and lumps formation. X-ray diffraction (XRD) data confirmed the synthesis of pristine HNPs having a rhombohedrally centered hexagonal structure. In addition, the particle size effect on the XRD peak shifting was studied and found to be consistent with the calculated size results by Scherer equation. Transmission electron microscopy revealed mixed polygonal and hexagonal morphologies with an average size of (16–44 nm) depending on the precursor concentration. Scanning electron microscopy depicts lots of agglomerations and lumps with increasing the precursor concentration. Raman spectra not only show the seven phonon modes of pristine hematite phase but also confirm the evolution of the crystal structure with concentration. Thermal analyses proved that the synthesized α-Fe2O3 products in the current work using 0.05 M precursor concentration are well crystallized at around ≈464 °C. Thus, current work presents smaller-sized HNPs crystallized at lower temperature ranges with different morphologies and higher probable vulnerable facets compared to previous studies. This will show improved properties and give rise to higher potentials for various applications. Furthermore, it verifies that the precursor nature is the key factor affecting both size and morphology, whereas, precursor concentration not only affects the particle size as reported previously but also crystallinity, morphology development, and lumps formation of the final product.

Improved size, morphology and crystallinity of hematite (α-Fe2O3) nanoparticles synthesized via the precipitation route using ferric sulfate precursor

Two different polymorphs of ferrous oxalate dihydrate were synthesized by precipitation of ferrous ions with oxalic acid: α-Fe(C2O4) · 2H2O with a monoclinic unit cell is obtained after precipitation and ageing at 90 °C, whereas the orthorhombic β-type is formed after precipitation at room temperature. The morphology of the oxalate crystals can be tailored from prismatic crystals of the α-polymorph over star-like aggregates of α/β-mixtures to non-agglomerated crystallites of β-oxalate. Thermal decomposition in air gives hematite at T ≥ 250 °C; if the thermolysis reaction is performed at low oxygen partial pressures (e.g., T = 500 °C and pO2 = 10−25 atm) magnetite is obtained. The synthesized magnetite is stoichiometric as signaled by lattice parameters of a0 = 8.39 A. The thermal decomposition of ferrous oxalate is monitored by thermal analysis, XRD, and IR-spectroscopy. The morphology of the oxalate crystals is preserved during thermal decomposition; the oxalates are transformed into spinel particle aggregates of similar size and shape. The crystallite size of the magnetite particles increases with temperature and is 40 or 55 nm, if synthesized from β-oxalate at 500 °C or 700 °C, respectively. The saturation magnetization of the magnetite particles decreases with decreasing particle size. Since the particles are larger than the critical diameter for superparamagnetic behavior they display hysteresis behavior at room temperature.

Synthesis of magnetite nanoparticles by thermal decomposition of ferrous oxalate dihydrate

This study has characterised the oxidation products of a fine-grained single domain magnetite which was made synthetically by a colloidal method. Changes in the intrinsic magnetic properties (saturation magnetisation, saturation remanence, and coercive force) during progressive oxidation are correlated with lattice parameter changes and an oxidation mechanism. It is proposed that magnetite oxidises to hematite via at least two metastable maghemites. The first of these, formed on low temperature oxidation by the formation of a magnetite/maghemite solid solution, is a face centered maghemite with lattice parameter a= 8.3419±0.0006 A. A second maghemite, produced on oxidation at higher temperatures, has a primitive cubic structure and a lattice parameter a = 8.3505±0.0005 A. Maghemite cation distributions are derived to explain the reduced saturation magnetisations of between 56 and 74 Am2 kg-1 observed, and a maghemite structure containing an increase in tetrahedral Fe3+ ions and up to 3 octahedral vacancies per 32 oxygen unit cell is proposed.

Saturation magnetisation, coercivity and lattice parameter changes in the system Fe3O4-γFe2O3, and their relationship to structure

Using a device for thermogravimetric analysis, a dynamic study of thermally induced solid-state transformations of FeC2O4·2H2O in the atmosphere allowing full participation of gaseous products (CO, CO2, H2O) in the reaction system was carried out. Solid phases formed at various temperatures between 25 and 640 °C were identified and characterized using 57Fe Mossbauer spectroscopy, TG and XRD. Up to 230 °C, evolution of two molecules of the water of crystallization takes place. Superparamagnetic nanoparticles of magnetite (Fe3O4) are formed as the primary product of the decomposition of FeC2O4, together with gaseous CO and CO2. In the next stage above 380 °C, the crystallization of magnetite is accompanied by a reduction of the remaining ferrous oxalate to iron carbide (Fe3C) by carbon monoxide. Thermally induced conversion of iron carbide into α-Fe and carbon is expected between 400 and 535 °C as the major chemical process. In the last reaction step, above 535 °C, magnetite is reduced to wustite (FeO) by carbon monoxide evolved at lower temperatures. On the grounds of quantitative Mossbauer data possible competitive reactions are discussed and a temperature dependent reaction model is suggested.

Thermal behaviour of iron(II) oxalate dihydrate in the atmosphere of its conversion gases

The thermal decomposition of ferrous oxalate dihydrate FeC2O4.2H2O, has been investigated using thermogravimetric (TG), differential thermogravimetric (DTG), differential thermal analysis (DTA), X-ray diffraction and infra-red spectroscopic techniques. The DTA data obtained in air consist of an endothermic dehydration peak at ∼ 190‡ C and two exothermic peaks at ∼ 245 and ∼ 360‡ C. The first exotherm corresponds to the primary oxidative decomposition while the second one seems to be due to a structural transformation from a disordered or non-crystalline to a crystalline state ofα-Fe2O3. Similar experiments carried out using a nitrogen atmosphere show an endothermic peak ∼ 190‡ C followed immediately by an exothermic peak at 240‡ C. The final product of this decomposition has been identified as Fe3O4. The oxalate dihydrate containing traces of moisture decomposes in air under the ambient of its own gaseous products at ∼ 300‡ C to giveγ-Fe2O3. This compound has been characterized by X-ray diffraction and magnetic hysteresis measurements.

A. B. Biswas

Papers

Studies on the formation of ?-Fe2O3 (maghemite) by thermal decomposition of ferrous oxalate dihydrate