Maghemite crystal structure

The optimal material for magnetic recording media was, for many years, y-FejOs (maghemite). This form of FejOj, which has a defect-spinel crystal structure, can be prepared as small, elongated particles. The coercive field is produced within the required range by shape anisotropy (see Section 4.5.1). The process used to prepare small particles of... 

All fine grained Fc oxides lose adsorbed water at characteristic temperatures of between 100 and 200 °C. Structural OH in gocthite and lepido-crocite is lost at 250-400°C by the dehydroxylation reaction 2OH O + H2O. Even fine grained oxides such as hematite contain some OH in the structure (Stanjek Schwertmann, 1992) and this is driven off over a wide temperature range. For Fe oxides endothermic peaks result from the release of adsorbed or structural water, whereas exothermic peaks come from phase transformations (e.g. maghemite to hematite) or from recrystallization of smaller crystals into larger ones. An example of this is observed during the transformation of ferrihydrite to hematite. 

Manganese oxides, which have different structural and surface properties, vary substantially in their ability to promote the precipitation and crystallization of Fe oxides and oxyhydroxides. The Mn(II) dissolved from Mn oxides in the presence of Fe(II) also influences the crystallization of oxidation products of Fe(II). The Fe oxides formed as influenced by Mn oxides and dissolved Mn(II) range from lepidocrocite, goethite, maghemite, dkaganeite, feroxyhyte, magnetite, honessite-like minerals, to noncrystalline Fe oxides. Therefore, Mn oxides deserve close attention in the genesis of Fe oxides. 

In a solid state reaction at low temperatures a solid reactant may be entirely converted to a solid product that keeps the original structure of the reactant. In such a thermal process new nonequilibrium solids can be made that cannot form when the system would be close to equilibrium. The temperature cannot be too low so that the reaction remains possible (sufficient mobility), and it should not be so high as to prevent crystallization to equilibrium structures. This procedure is called chimie douce or soft chemistry. An example of chimie douce from industrial chemistry is the oxidation of magnetite (a mixed valence form of iron oxide, FCjOJ to the ferromagnetic y-Fe203 (maghemite) in a microstructure that renders it suitable for... 

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