Hematite magnetic hyperfine field

Fig. 3.9 Effect of Al-substitution in synthetic hematites on (Left) the unit cell edge length a of hematites synthesized at various temperatures (Stanjek Schwertmann, 1992, with permission), and (Right) the magnetic hyperfine field Bhf of hematites formed at 70 °C and 1000°C dotted lines indicate 95% confidence limits (Murad Schwertmann 1986 with permission).

Morris et al. (1991) obtained hematite of very small particle size ( 10 nm), termed nanophase by slow thermal decomposition in air of tri-Ee -acetato-hy-droxy-nitrate. XRD shows only two broad lines as in a 2-line ferrihydrite, but the magnetic hyperfine field at 4.2 K of 50.4 T appears to be more in agreement with poorly crystalline hematite. Well-crystalline hematite and Al-hematite were produced by decomposing Ee-Al-oxinates at 700 °C (da Costa et al. 2001). 

Method 3 produces ca. 3 g of hematite. The sample prepared in 2 10 M HCl consists of subrounded crystals between 30-50 nm across (Fig. 10-1 c) with a surface area of around 30 m /g. In 10 M HCl the crystal size is around 150-200 nm (Fig. 10-1 d) and the surface area is only a few m /g. The X-ray peaks of the 0.002 M HCl product are somewhat broader than those of the material produced in 10 M HCl owing to the smaller crystal size. The Mossbauer spectrum at RT (Fig. 10-4) shows a sextet corresponding to a magnetic hyperfine field of 53.3 T. 

S isomer shift reiative to a-Fe at room temperature, 2s quadrupole shift, A quadrupole splitting, 6 magnetic hyperfine field, f. Mdssbauer fraction relative to that of hematite [13,19],... 

Similarly as for goethite, a linear relationship of R as a function of A1 content and particle size has been proposed [ 102] which is valid at RT and for concentrations less than 10 %A1. However, all these results are derived from synthetic samples, mostly obtained from goethite. Therefore, particularly at RT, the magnetic hyperfine field will still be largely influenced by morphological effects. Moreover, most preparation methods, based on the decomposition of oxyhydroxides, result in inhomogeneous A1 substitution [96]. A more clear-cut picture for the dependence of the hyperfine field on A1 substitution is obtained for hematites prepared from oxinates [103] where a reduction of 0.061 at RT and 0.032 at 80 K per at % A1 is observed. 

Mossbauer spectroscopy is also able to give local moment orientations, with respect to the crystalline lattice, or the correlations between moment orientations and local distortion axis orientations in a chemically disordered or amorphous material. This arises from the interplay between the structural (electric field gradient) hyperfine parameters and the magnetic hyperfine parameters. In this way, the spin flop Morin transition of hematite, for example, is easily detected and characterized (e.g., Dang et al. 1998). The noncollinear magnetic structures of nanoparticles can also be characterized. 

At pH 2.5 the oxide surface is positively charged and ferric ions are present in dilute solution (lO moir ) as [Fe(OH2)f,] (70%), [Fe(OH)(OH2)5] + (20%), [Fe(OH)2(OH2)4] (3%) and [Fe2(OH)2(OH2)5] (6%). Mossbauer spectra of the suspension (Figure 9.3a), which exhibit a sexluplet, show that all adsorbed ferric ions interact magnetically with the substrate. The parameters of the adsorbed ions (isomeric shift, average hyperfine field) are very similar to those of solid hematite. Within the adsorbed layer, ferric ions occupy sites characteristic of the stmcture of hematite, and hence the layer is an extension of the crystalline network of the... 

The refined spectral parameters for the individual components observed in the room-temperature (300 K) Mossbauer effect experiments, including the spectral area (Arei), isomer shift (IS), quadrupole splitting (QS) as well as the hyperfine magnetic field (fihr), are listed in Table 4.29. The hyperfine parameters for the identified components (hematite, magnetite, goethite, lepidocrocite and feroxyhyte) are listed in [215]. 

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