Iron(II,III) oxide
Iron(II,III) oxide is the chemical compound with formula FeO. It occurs in nature as the mineral magnetite. It is one of a number of iron oxides, the others being iron(II) oxide (FeO), which is rare, and iron(III) oxide (FeO) which also occurs naturally as the mineral hematite. It contains both Fe and Fe ions and is sometimes formulated as FeO ∙ FeO. This iron oxide is encountered in the laboratory as a black powder. It exhibits permanent magnetism and is ferrimagnetic, but is sometimes incorrectly described as ferromagnetic. Its most extensive use is as a black  pigment. For this purpose, it is synthesized rather than being extracted from the naturally occurring mineral as the particle size and shape can be varied by the method of production. Preparation. Under anaerobic conditions, ferrous hydroxide (Fe(OH)) can be oxidized by water to form magnetite and molecular hydrogen. This process is described by the Schikorr reaction: The well-crystallized magnetite (FeO) is thermodynamically more stable than the ferrous hydroxide (Fe(OH) ). Magnetite can be prepared in the laboratory as a ferrofluid in the Massart method by mixing iron(II) chloride and iron(III) chloride in the presence of sodium hydroxide. Magnetite can also  be prepared by the chemical co-precipitation in presence of ammonia, which consist in a mixture of a solution 0.1 M of FeCl·6HO and FeCl·4HO with mechanic agitation of about 2000 rpm. The molar ratio of FeCl:FeCl can be 2:1; heating this solution at 70 °C, and immediately the speed is elevated to 7500 rpm and adding quickly a solution of NHOH (10 volume %), immediately a dark precipitate will be formed, which consists of nanoparticles of magnetite. In both cases, the precipitation reaction rely on a quick transformation of acidic hydrolyzed iron ions into the spinel iron oxide  structure, by hydrolysis at elevated pH values (above ca. 10). Considerable efforts has been devoted towards controlling the particle formation process of magnetite nanoparticles due to the challenging and complex chemistry reactions involved in the phase transformations prior to the formation of the magnetite spinel structure. Magnetite particles are of interests in bioscience applications such as in magnetic resonance imaging (MRI) since iron oxide magnetite nanoparticles represent a non-toxic alternative to currently employed gadolinium-based contrast agents. However, due to lack of control over the specific transformations involved in the formation of the particles, truly superparamagnetic particles have not  yet been prepared from magnetite, i.e. magnetite nanoparticles that completely lose their permanent magnetic characteristic in the absence of an external magnetic field (which by definition show a coercivity of 0 A/m). The smallest values currently reported for nanosized magnetite particles is "Hc" = 8.5 A m, whereas the largest reported magnetization value is 87 Am kg for synthetic magnetite. Pigment quality FeO, so called synthetic magnetite, can be prepared using processes that use industrial wastes, scrap iron or solutions containing iron salts (e.g. those produced as by-products in industrial processes such as the acid vat treatment (pickling ) of steel): Reduction of FeO with hydrogen: Reduction of FeO with CO: Production of nano-particles can be performed chemically by taking for example mixtures of Fe and Fe salts and mixing them with alkali to precipitate colloidal FeO. The reaction conditions are critical to the process and determine the particle size. Reactions. Reduction of magnetite ore by CO in a blast furnace is used to produce iron as part of steel production process: Controlled oxidation of FeO is used to produce brown pigment quality γ-FeO (maghemite): More vigorous calcining (roasting in air) gives red pigment quality α-FeO  (hematite): Structure. FeO has a cubic inverse spinel group structure which consists of a cubic close packed array of oxide ions where all of the Fe ions occupy half of the octahedral sites and the Fe are split evenly across the remaining octahedral sites and the tetrahedral sites. Both FeO and γ-FeO have a similar cubic close packed array of oxide ions and this accounts for the ready interchangeability between the three compounds on oxidation and reduction as these reactions entail a relatively small change to the overall structure. FeO samples can be non-stoichiometric. The ferrimagnetism of FeO  arises because the electron spins of the Fe and Fe ions in the octahedral sites are coupled and the spins of the Fe ions in the tetrahedral sites are coupled but anti-parallel to the former. The net effect is that the magnetic contributions of both sets are not balanced and there is a permanent magnetism. In the molten state, experimentally constrained models show that the iron ions are coordinated to 5 oxygen ions on average. There is a distribution of coordination sites in the liquid state, with the majority of both Fe and Fe being 5-coordinated to oxygen  and minority populations of both 4- and 6-fold coordinated iron. Properties. FeO is ferrimagnetic with a Curie temperature of 858 K. There is a phase transition at 120K, called Verwey transition where there is a discontinuity in the structure, conductivity and magnetic properties. This effect has been extensively investigated and whilst various explanations have been proposed, it does not appear to be fully understood. While it has much higher electrical resistivity than iron metal (96.1 nΩ m), FeO's electrical resistivity (0.3 mΩ m ) is significantly lower than that of FeO (approx kΩ m). This is ascribed to  electron exchange between the Fe and Fe centres in FeO. Uses. FeO is used as a black pigment and is known as "C.I pigment black 11" (C.I. No.77499) or Mars Black. FeO is used as a catalyst in the Haber process and in the water-gas shift reaction. The latter uses an HTS (high temperature shift catalyst) of iron oxide stabilised by chromium oxide. This iron–chrome catalyst is reduced at reactor start up to generate FeO from α-FeO and CrO to CrO. Bluing is a passivation process that produces a layer of FeO on the surface of steel  to protect it from rust. Ferumoxytol along with sulfur and aluminium, it is an ingredient in a specific type of thermite useful for cutting steel. Medical uses. Nano particles of FeO are used as contrast agents in MRI scanning. Ferumoxytol, sold under the brand names Feraheme and Rienso, is an intravenous FeO preparation for treatment of anemia resulting from chronic kidney disease. Ferumoxytol is manufactured and globally distributed by AMAG Pharmaceuticals. Biological occurrence. Magnetite has been found as nano-crystals in magnetotactic bacteria (42–45 nm) and in the beak tissue of homing pigeons.