Substituted Magnetite

The fact that the temperature domain of positive Seebeck coefficients below Ty, see Fig. 6, is wiped out by x = 0.01 shows that a donor state associated with an F ion lies above the top of the FeB-ai(J,) valence band in the low-temperature phase it charge compensates the holes in that band. This, in turn, means that the clusters of an F ion 

On the other hand, above Tv the Seebeck coefficient remains negative despite the increasing density of Fe ions, see Fig. 5. This can be attributed to two factors  

The tail states associated with F -ion substitutions also introduce the possibility of variable-range hopping and hence a In a versus dependence. Indeed, Graener et 

Further evidence for trapped Fel ions comes from Mossbauer data taken at 300 K and 77 which show identifyable FeB ions the intensity of the weak peak (rela- 

Whether a solute cation substitutes for A-site or B-site iron depends upon the relative site-preference energies of the ions. Moreover, a solute atom may lower the energy A of Fig. 3 to where significant concentrations of FeA ions are stabilized in the presence of Fcb ions at room temperature. Temperature-dependent cation distributions can be expected where the relative site-preference energies differ by a Ae kT. 

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Fig. 3.2 Fraction of various metals released versus Fe released during acid dissolution of synthetic metal-substituted magnetites (upper six plots Sidhu et al., 1978, with permission), goethites and hematites (lower plots Lim-Nunez dikes, 1987 with permission).

Substituted magnetite Dissolution/reprecipitation Alkaline solution with M ... 

Burnishing is the formation of black-brown oxide films on iron and its alloys by controlled oxidation of cleaned metal surfaces. These films are extremely complex and contain, in addition to maghemite and magnetite (or a substituted magnetite for Ni, Mo or Co alloys), various nitride phases - Fe4N, FeaN and FeN. The nitride phases are adjacent to the metal and the iron oxides are in the outer layers of the film (Gebhardt, 1973). 

Schwertmann, U. Murad, E. (1990) The influence of aluminum on iron oxides. XIV. Al-substituted magnetite synthesized at ambient temperatures. Clays Clay Min. 38 196-202... 

A lower degree of A1 substitution can be obtained by this method with a lower initial Al/(A1+Fe) ratio. However, under these conditions, A1 substituted magnetite is formed in addition to goethite (Schwertmaim and Murad, 1990). The goethite may be purified by extracting the magnetite with M H2SO4 at 80 °C. 

Fig. 11. Mossbauer spectra recorded at 298 K of (from top to bottom) magnetite Al-substituted magnetite with 7 mol% Al maghemite produced by heating pure magnetite at 250 X.

The effect of Si substitution on the turnover frequency for WGS is shown in Figure 11. The turnover frequencies plotted in this figure were based on the magnetite surface area as determined by the NO chemisorption technique. The turnover frequencies shown for unsupported Fe O indicate that the factor of 10 decline in activity for the silica-supported catalysts is not a particle size effect, but instead is a consequence of the substitution of Si into the lattice. However, when the adsorption of CO/COo at 663 K was used to titrate the surface sites instead of NO, the resulting turnover frequencies were essentially constant as shown in Figure 12. Accordingly, the CO/CO2 mixture apparently titrates the sites active for WGS. Clearly, the number of active sites is decreased markedly as the particle size decreases in the silica-substituted magnetite catalysts. 

Bickford, L. R., Pappis, J. Stull, J. L. (1955). Magnetostriction and permeability of magnetite and cobalt-substituted magnetite. Physical Review, 99, 1210-14. 

Nakagawa, Y., Hori, H., Goto, T. Miyata, N. (1982). Magneto-optical spectra of zinc-, manganese- and nickel-substituted magnetite. In Ferrites Proceedings of the Third International Conference on Ferrites, Kyoto, 1980. Eds H. Watanabe, S. lida and M. Sugimoto. Center for Academic Publications, Tokyo, pp. 110-14. 

Crucial to the understanding of metal-substituted magnetite and their properties is the determination of the site occupancy of the metals. The structure of magnetite is very well known. It is a cubic, inverse spinel with one quarter of the tetrahedral (Tj) and one half of the octahedral (0 j) sites filled by Fe. The formula for magnetite is (Fe )[Fe Fe ]04 where Fe ... 

L Diamandescu, D Mihaila-Taraba anu, V Teodorescu, N Popescu-Pogrion. Hydrothermal synthesis and structural characterization of some substituted magnetites. Mater Lett 1998 37 340. 

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