Magnetite inverse spinel structure

The sensitive dependence of the electrical and magnetic properties of spinel-type compounds on composition, temperature, and detailed cation arrangement has proved a powerful incentive for the extensive study of these compounds in connection with the solid-state electronics industry. Perhaps the best-known examples are the ferrites, including the extraordinary compound magnetite Fc304 (p. 1080) which has an inverse spinel structure (Fe )t[Fe Fe ]o04. 

The mineral magnetite is a naturally occurring form of Fe304 that has an inverse spinel structure as a result of both Fe2+ and Fe3+ being present. 

For example, the inverse spinel structure of magnetite (see Chap. 2) results from the fact that the CFSE of Fe is greater for octahedral than for tetrahedral coordination, so Fe preferentially occupies octahedral sites. For Fe the CFSE is zero for both octahedral and tetrahedral coordination, so that this ion has no preference for either type of coordination. 

Ferrite compounds with the inverse spinel structure are similar to magnetite, with different ions substituting for the iron atoms. As with FeO (cf. Figure 6.62), the oxygen ions have no permanent magnetic moment. Tetrahedral sites in the FCC oxygen array are occupied by half of the trivalent cations, and octahedral sites are occupied equally by divalent cations and the remaining trivalent cations. 

The saturation magnetization of magnetite is 5.2xl05 Am-1 and the unit cell is of side 837 pm. Assuming the inverse spinel structure, estimate the magnetic moment (in Bohr magnetons) of the Fe2+ ion. [Answers 6.7 fiB and 4 fiB 4.11 B]... 

Magnetite (FC3O4) is a component of the water-gas shift reaction that crystallizes to the inverse spinel structure. The general formula of the oxides known as spinel is AB204. In the normal spinel-type structure, A is an A2+ metal, and B is a B3+ metal. O2- forms a CCP anionic framework, where the A atoms occupy 1/8 of the tetrahedral sites and the B atoms occupy 1/2 of the octahedral sites (see Figures 1.6 and 1.7). An example of a normal spinel is Mg A1204. 

Magnetic iron oxide, Fe304, occurs naturally as the mineral magnetite. It has an inverse spinel structure (see Chapter 9) because it contains Fe2+ and Fe3+, and the formula can be written as FeO Fe203. 

In the inverse spinel structure the A11 ions and one-half of the B111 ions have exchanged place that is the A11 ions occupy octahedral holes along with one-half of the Bm ions while half of the Bm ions are in tetrahedral holes. Fe304 (magnetite) is an example of an inverse spinel structure. Although both A and... 

The inverse spinel structure of magnetite can be described as follows. Starting with a ccp arrangement of ions, one-quarter of the octahedral holes are filled with Fe ions and one-quarter with Fe ions one-eighth of the tetrahedral holes are occupied with Fe ions. Show that this corresponds to a formula of Fe304, and that the compound is charge-neutral. 

Finally, there is Fe304, a mixed Fc,l-Fe"1 oxide which occurs in Nature in the form of black, octahedral crystals of the mineral magnetite. It can be made by ignition of Fe203 above 1400°. It has the inverse spinel structure... 

Magnetite possesses an inverse spinel structure with oxygen ions forming a face-centred cubic closely packed structure. The formula for describing Fe occupancy is (Fe " ) [Fe ", Fe ]04 where the parentheses ( ) stand for cations at tetrahedral sites while brackets [ ] denote cations at octahedral lattice sites. Stoichiometric magnetite has all available substitutional sites occupied by Fe and Fe ions. Non-stoichiometric magnetites also exist, with various numbers of available sites being either vacant or occupied by impurity ions. 

Following correct reduction, the iron oxide/chromium oxide catalyst consists largely of magnetite, which has an inverse spinel structure (231) in which the... 

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