SUMMARY Raman spectroscopy uses the inelastic scattering of electromagnetic radiation by molecules. Monochromatic light of a laser interacts with phonons, the vibrational modes in the crystal lattice. The energy of the scattered light is shifted by the scattering. The shifts in energy yield the Raman spectrum that is specific for each mineral because the phonons are specific for each mineral. In this study, Raman spectroscopy of synthetic and natural iron (oxy)hydroxides and iron oxides was performed to test its potential in environmental magnetic studies and soil science. The main aim was to distinguish between the different iron oxides occurring in soils. Most of them can be identified by magnetic methods, but there are some minerals that are not easy to differentiate from each other. In these cases, the magnetic methods can be complemented by Raman spectroscopy. A major challenge is the fast transformation of many iron minerals if laser power is applied, especially if the material is poorly crystallized as often is the case in environmental material. In this study, very low laser powers were applied. Nevertheless, the investigated iron minerals could be distinguished from each other. Thus, a magnetic method to discern lepidocrocite and ferrihydrite in soil samples could be corroborated. It is also shown that Raman spectroscopy is an easy method to distinguish magnetite and maghemite. Due to the low laser powers applied, a wuestite band at about 595 cm −1 could be established enabling a non-ambiguous identification of this mineral by its Raman spectrum. Furthermore, the potential of the method to investigate magnetic material produced by soil bacteria is demonstrated.

/pdf/raman-spectroscopy-of-iron-oxides-and-oxy-hydroxides-at-low-59ffjtcrp9.pdf

Raman spectroscopy of iron oxides and (oxy)hydroxides at low laser power and possible applications in environmental magnetic studies

Thin films of three iron oxide polymorphs, hematite, maghemite, and magnetite, were produced on KBr substrates using a conventional electron beam deposition technique coupled with thermal annealing. This method allowed for iron oxide thin films free from chemical precursor contaminants. The films were characterized using Fourier-transform infrared spectroscopy (FTIR), Raman microspectroscopy, and ellipsometry. These spectroscopic techniques allowed for a clear assignment of the phase of the iron oxide polymorph films produced along with an examination of the degree of crystallinity possessed by the films. The films produced were uniform in phase and exhibited decreasing crystallinity as the thickness increased from 40 to 250 nm.

Vibrational Spectroscopic Characterization of Hematite, Maghemite, and Magnetite Thin Films Produced by Vapor Deposition

Structural and magnetic properties, methods of synthesis, and applications of seven iron(III) oxide polymorphs, including rare beta, epsilon, amorphous, and high-pressure forms, are reviewed. Thermal transformations resulting in the formation of iron oxides are classified according to different parameters, and their mechanisms are discussed. 57Fe Mossbauer spectroscopy is presented as a powerful tool for the identification, distinction, and characterization of individual polymorphs. The advantages of Mossbauer spectroscopy are demonstrated with two examples related to the study of the thermally induced solid-state reactions of Fe2(SO4)3.

Iron(III) Oxides from Thermal ProcessesSynthesis, Structural and Magnetic Properties, Mössbauer Spectroscopy Characterization, and Applications†

Films and powders of iron oxide (Fe2O3) prepared by two different sol–gel syntheses, starting from Fe(NO3)3·9H2O or FeCl3·6H2O, were investigated by Raman microscopy. Different phases with different morphology were produced according to the preparation. The spectra obtained in the micro-Raman configuration were compared with the ambiguous data in the literature given by conventional Raman techniques. The principal difficulty in the correct interpretation of the Raman spectrum of the iron oxides is the co-existence of different phases. Contradictory results are also explained by laser–induced thermal effects which easily change the wavenumbers and lineshapes of the phonons. A Stokes–Anti-Stokes procedure was utilized to control the local temperature during the measurement and also for calibration of the wavenumbers. The micro-Raman spectra of hematite, magnetite and other iron oxides are presented and compared with literature data. Copyright © 1999 John Wiley & Sons, Ltd.

Micro‐Raman investigation of iron oxide films and powders produced by sol–gel syntheses

https://digital.csic.es/bitstream/10261/75594/4/Long_term_corrosion....pdf

Long-term atmospheric corrosion of mild steel

For fundamental studies of the atmospheric corrosion of steel, it is useful to identify the iron oxide phases present in rust layers. The nine iron oxide phases, iron hydroxide (Fe(OH)2), iron trihydroxide (Fe(OH)3), goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), feroxyhite (δ-FeOOH), hematite (α-Fe2O3), maghemite (γ-Fe2O3) and magnetite (Fe3O4) are among those which have been reported to be present in the corrosion coatings on steel. Each iron oxide phase is uniquely characterized by different hyperfine parameters from Mossbauer analysis, at temperatures of 300K, 77K and 4K. Many of these oxide phases can also be identified by use of Raman spectroscopy. The relative fraction of each iron oxide can be accurately determined from the Mossbauer subspectral area and recoil-free fraction of each phase. The different Mossbauer geometries also provide some depth dependent phase identification for corrosion layers present on the steel substrate. Micro-Raman spectroscopy can be used to uniquely identify each iron oxide phase to a high spatial resolution of about 1 µm.

Characterization of Iron Oxides Commonly Formed as Corrosion Products on Steel

Atmospheric corrosion of different steels in marine, rural and industrial environments

The relative recoilless fraction (F-value) of each of six iron oxides, defined as the ratio of the recoil-free fractions of two different materials, was experimentally determined relative to hematite at 300 K and 77 K by Mossbauer spectroscopy. Using the relative recoil-free fractions compared to that of hematite, the relative recoilless fractions between all pairs of the seven iron oxides were determined. The F-values can allow conversion of Mossbauer subspectral areas to the relative atomic, molecular, or weight fractions of each iron oxide present in a mixed oxide phase sample.

Mössbauer effect determination of relative recoilless fractions for iron oxides

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Characterization of Iron Oxides Commonly Formed as Corrosion Products on Steel

Atmospheric corrosion of different steels in marine, rural and industrial environments

Mössbauer effect determination of relative recoilless fractions for iron oxides