Magnetite unit cell

Fig. 2.13 Structure of magnetite, a) Polyhedral model with alternating octahedra and tetrahedra-octahedra layers, b) Ball-and-stick model. Unit cell outlined, c) Ball-and-stick model ofthe arrangement of octahedra and tetrahedra. (a-c Stanjek, unpubl.)...

There are structural analogues of a number of iron oxides in the Fe-H-O system. Under certain conditions, continuous solid solutions exist between the two members of a pair. The magnetite-ulvospinel and the hematite-ilmenite pairs are well-known examples. The principle in going from the Fe oxide to the Ti-containing phase is to replace two Fe by one Fe" and one Ti , thereby increasing the unit cell size. 

Fig. 14.25 Change in degree of oxidation and unit cell size of synthetic and natural magnetites with time of heating. (The time scale is in minutes for synthetic and in hours for natural magnetites) (Sidhu et al., 1981 a with permission).

Two different pathways of formation are possible (Stanjek, 2000). One route involves aerial oxidation of lithogenic magnetite as suggested for Brazilian Oxisols on basic igneous rocks. The mechanism of this topotactic reaction is described in Chapter 14. These maghemites are usually titaniferous as are the magnetites from which they are derived (see Chap. 15) and almost free from or very low in Al (Allan et al., 1989). Their unit cell size is a function of the residual Fe" and the Ti content. 

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.

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]... 

The composition of the resulting magnetite is close to stoichiometric as indicated by chemical analysis (Fe2,o8Feo.9204), Mossbauer spectroscopy (Fe2,o3Feo.9704), and unit cell edge length (0.83997(3) nm). The crystals form cubes, bounded by 111 faces and vary in size between 0.05-0.2 p.m (Fig. 11-2) the surface area is 4 m /g. The XRD lines are sharp... 

FeCb (instead of FeS04) also yields magnetite. Oxidation at RT instead of at 90 C gave smaller crystals (ca. 50 nm) which chemical analysis, unit cell size and magnetic hyperfine properties showed to be partially... 

Magnetite, Fes04, is the outstanding example of this class of semi-conductor. De Boer and Verwey (7) first proposed the interpretation of its properties and Verwey and his co-workers (8) have studied the structural problem with some success. They deduce from their studies of related spinels that, at ordinary temperature, F O has all the Fe + ions and half the Fe + ions distributed statistically over one kind of cation position (octahedral) in the unit cell. The remaining Fe + are localized in tetrahedral sites. Ihe ease of electron transfer from Fe + to Fe ions is responsible for the... 

Magnetite is an example of naturally occurring ferrite. It has spinel structure. The unit cell of spinel ferrite consists of 32 oxygen, 16 trivalent iron and 8 divalent iron atoms. The most important feature of tmit cell is that its array of oxygen ions leaves two open kinds of interstices which can be filled by metal ions. These interstices are referred to as tetrahedral or A sites and octahedral or B sites. 

Magnetite structure is an inverse spinel with a face center cubic unit based on 32 O2- ions with a regularly cubic dose packed along the [111] direction. There are eight formula units per unit cell. Magnetite differs from other iron oxides in that it contains both divalent and trivalent iron. Its formula is written as Fe > [Fe Fe in]04and the brackets denote octahedral sites, tetrahedral Fe spans are directed antiparallel to octahedral Fe 3+ and Fe 2+ spins so... 

Magnetite has a facecubic unit cell where tetrahedral sites are occupied by Fe + ions, while both Fe and Fe ions can be found at octahedral sites. 

Figure 7. Neutral stacking sequence generating magnetite parallel to (001) large medium gray atoms are oxygens, small light gray atoms are octahedral irons, small dark atoms are tetrahedral irons. Bulk unit cell is shown by the solid box the surface unit cell is shown by the dashed box.

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