The magnetism of fine particle iron oxides and oxyhydroxides in applied fields

Mössbauer Characterization and in Situ Monitoring of Thermal Decomposition of Potassium Ferrate(VI), K2FeO4 in Static Air Conditions

Solid orthorhombic crystals of potassium ferrate(VI) (K(2)FeO(4)) of a high-chemical purity (>99.0%) were characterized by low-temperature (1.5-5 K), high-temperature (463-863 K), and in-field (1.5 K/3 T) Mössbauer spectroscopy. Potassium ferrate(VI) reveals a Néel magnetic transition temperature (TN) of approximately 3.8 K and a saturation hyperfine magnetic field of 13.8 T at 1.5 K. Spectral line intensities recorded below TN in an external magnetic field of 3 T manifest a perfect antiferromagnetic ordering. For the in situ monitoring of the thermal behavior of K(2)FeO(4), high-temperature Mössbauer data were combined with those obtained from thermogravimetry, differential scanning calorimetry, and variable-temperature X-ray diffraction measurements. Such in situ approach allowed the identification of the reaction products and intermediates and yielded the first experimental evidence for the participation of CO2 in the decomposition process. As the primary conversion products, KFeO(2) and two potassium oxides in equivalent molar ratio, KO2 and K(2)O, were suggested. However, the KO2 phase is detectable with difficulty as it reacts very quickly with CO2 from air resulting in the formation of K(2)CO(3). The presented decomposition model is consistent with thermogravimetric data giving the mass loss of 8.0%, which corresponds to the participation of 1/6 mol of CO2 and liberation of 3/4 mol of O2 per 1 mol of K(2)FeO(4) (K(2)FeO(4) + 1/6CO2 --> KFeO(2) + 1/3K(2)O + 1/6K(2)CO(3) + 3/4O2). An explanation of the multistage reaction mechanism has an important practical impact for the optimization of the solid-state synthesis of potassium ferrate(VI).

Amorphous Iron(III) OxideA Review

The syntheses of amorphous Fe(2)O(3) nanoparticles of varying size and morphology, their magnetic properties, crystallization mechanism, and applications are reviewed herein. The synthetic routes are classified according to the nature of the sample (powders, nanocomposites, films, coated particles). The contributions of various experimental techniques to the characterization of an amorphous Fe(2)O(3) phase are considered in this review, including some key experimental markers, allowing its distinction from nanocrystalline "X-ray amorphous" polymorphs (maghemite, hematite). We discuss the thermally induced crystallization mechanisms depending on transformation temperature, atmosphere, and the size of the amorphous particles that predetermine the structure of the primarily formed crystalline polymorph. The controversial description of the magnetic behavior, including an interpretation of the low-temperature and in-field Mössbauer spectra, is analyzed.