Spinel structure, magnetite

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. 

At room temperature, magnetite has the cubic-spinel structure is a good electronic conductor, and exhibits a spontaneous magnetism. 

Figure 1.41) have the oxygen ions in a nearly close-packed cubic array. The unit cell contains 32 oxygen ions, with 32 octahedral and 64 tetrahedral sites, of which 16 of the octahedral and 8 of the tetrahedral sites are filled. It is the position of these 24 cations within the unit cell that determines magnetic behavior. The distribution of cations in the sites is specific to the type of cations, and it must be determined experimentally. There are two idealized spinel structures. In the normal spinel, all the divalent ions are on the tetrahedral sites, as in ZnFe204. In the inverse spinel, the 8 occupied tetrahedral sites are filled with trivalent ions and the 16 occupied octahedral sites are equally divided between di- and trivalent ions (see Figure 6.63). The prototypical inverse spinel ferrite is magnetite, whose structure consists of an FCC oxygen array with Fe + and Fe + ions in the interstices.

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. 

Fe304, magnetite [1309-38-2], spinel structure, ferrimagnetic, color black... 

Just as vivianite is regarded as the simplest example of a mineral with isolated clusters of Fe2+-Fe3+ octahedra showing IVCT transitions, so too is magnetite considered to be the classic example of a structure-type with infinite chains of Fe2+ -Fe3+ octahedra exhibiting electron delocalization. Magnetite, Fe304 or Fe2+Fe3+204, has the spinel structure illustrated in fig. 4.18 with an inverse cation distribution ( 6.4). Thus, half the Fe3+ ions occupy isolated tetrahedral sites,... 

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. 

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