A-FeOOH samples

Mossbauer spectra, hyperfine magnetic field distributions, and selected FE-SEM images of a-FeOOH samples obtained after various aging times in a highly alkaline medium in the presence of TMAH. (Reproduced from Ref. 336 with permission of Elsevier.)... 

Sn doped a-FeOOH samples of various crystallinity have been investigated using Mossbauer spectroscopy by Berry et al. [317]. The lines in the Mossbauer spectra recorded at room temperature were broadened and the HMF values were decreased due to Sn ion incorporation and deviation from perfect crystallinity. The spectra recorded at 80K were less affected by differences in crystallinity of samples, while the spectra recorded at I7K were almost unaffected. 

Fig. XVII-31. (a) Nitrogen adsorption isotherms expressed as /-plots for various samples of a-FeOOH dispersed on carbon fibers, (h) Micropore size distributions as obtained by the MP method. [Reprinted with permission from K. Kaneko, Langmuir, 3, 357 (1987) (Ref. 231.) Copyright 1987, American Chemical Society.]...

To date the double extrapolation technique has only been applied to ionizable latexes (19). Since this technique avoids some of the assumptions of our earlier methods (3 ), it would also be useful for a determination of the intrinsic surface stability constants of oxides. Figures 2 and 3 show sample calculations of and for a-FeOOH in NaCl, using the experimental... 

The chemical composition of different nodules used in the present study is widely varied (See Table 1). XRD patterns showed a few diffused peaks characteristic of S-MnOj, a-FeOOH and a-SiOj. As expected the samples with high silica contents showed more surface area than the samples with lower silica contents. The surface oxygen, however, does not vary widely. 

The inconsistencies were explained by Dezsi and Fodor [43], who found that three out of five deposits of natural a-FeOOH and a synthetic sample all gave a unique hyperfine pattern, while the other two did show clear evidence of two fields. The latter result was found to be true of samples containing excess water deposits of stoichiometric a-FeOOH do not show evidence for more than two magnetic sublattices. The Neel temperature was found to be only 367 K, and relaxation occurred at up to 30° below this. It would appear that the defect structure and particle size are both important in determining the properties of a specific sample. 

Figure 5. Distribution of particle-size parameter (average diameter of cross-sectional area) for geothite (a-FeOOH) in samples of Middle Kittanning coal. Key A, high wall sample O, middle sample , outcrop sample and P, original pyrite sample. See Table / for definition of class limits.

Figure19.2 (a) Scanning electron microscopy image ofthe as-prepared sample. In addition to goethite a-FeOOH nanowires, large micrometer-sized c stallites of a-Fe203 (hematite) were found (b) Transmission electron microscopy (TEM) image ofthe sample after sonication where hematite c stallites were...

Influence of Trivalent Cations Due to the common presence of Al-substituted a-FeOOH in natural samples (well oxidized soils), the Impact of Al-substitution on the properties of natural and synthetic goethite was... 

Fig. 4.28. Simulated Mossbauer spectra for microcrystalline samples of the uniaxial material a-FeOOH with a broad particle size distribution. The effect of pronounced collective excitations are seen in (a) where the mean value of 1cbT/XF 0.1S while the effect is much reduced in (h) where the mean value of kBT/KF-0.04.

The verification of the presence of hydrogen in the film has proved more controversial, primarily because many of the structural investigations have been carried out after the film has been dried in vacuo. An example of the problems here is the fact that electron diffraction, which has to be carried out in vacuo, reveals a relatively well-crystallised spinel lattice whose origin may be the comparatively high sample heating encountered in the electron beam. Moreover, the use of in situ techniques, such as Mossbauer and X-ray absorption spectroscopy, clearly reveals marked differences between the spectra of the films in situ and the spectra of the same films ex situ as well as the spectra of y-Fe203 and y-FeOOH standards. These differences are most naturally ascribed to hydration of the spinel forms. 

Synthetic 5-FeOOH has a surface area which ranges from 20-300 m g depending on the thickness of the crystals. In a series of seven synthetic feroxyhytes the surface area increased from 140 to 240 m g (EGME method) as the crystallinity decreased (Garlson and Schwertmann, 1980). 5-EeOOH displays interpartide porosity, i.e. slitshaped micro- or mesopores between the plate like crystals (Jimenez-Mateos et al., 1988 Ishikawa et al., 1992). Both TEM observations and t-plot analysis showed that 0.8 nm micropores formed upon dehydroxylation at 150 °G in vacuo. The surface area rose steeply as the temperature exceeded 100 °G and reached a value close to 150 m g at 200 °C at which temperature, the sample was completely converted to hematite. 

A common feature of the dehydroxylation of all iron oxide hydroxides is the initial development of microporosity due to the expulsion of water. This is followed, at higher temperatures, by the coalescence of these micropores to mesopores (see Chap. 5). Pore formation is accompanied by a rise in sample surface area. At temperatures higher than ca. 600 °C, the product sinters and the surface area drops considerably. During dehydroxylation, hydroxo-bonds are replaced by oxo-bonds and face sharing between octahedra (absent in the FeOOH structures see Chap. 2) develops and leads to a denser structure. As only one half of the interstices are filled with cations, some movement of Fe atoms during the transformation is required to achieve the two thirds occupancy found in hematite. 

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