Structural and Magnetic Properties of Sn-, Ti-, and Mg-Substituted α-Fe2O3: A Study by Neutron Diffraction and Mössbauer Spectroscopy

Rietveld structure refinement of the X-ray powder diffraction data recorded from tin-doped Fe
 3
O
 4
 prepared by heating metallic iron, α-iron(III) oxide and tin(IV) oxide in sealed evacuated quartz ampoules shows the tin to substitute on the octahedral sites of the inverse spinel-related structure. Tin-doped γ-Fe
 2
O
 3
, prepared by boiling a precipitate derived from iron(III), iron(II) and tin(II) under reflux, also adopts an inverse spinel-related structure containing tin on the octahedral sites and, compared to pure γ-Fe
 2
O
 3
, is stabilised with respect to conversion to α-Fe
 2
O
 3
. 57
Fe Mossbauer spectroscopy shows the Neel temperature for γ-Fe
 1.9
Sn
 0.1
O
 3
 to be between 750 and 820 K. Tin-doped α-Fe
 2
O
 3
 prepared by hydrothermal methods is shown by Rietveld structure refinement of the X-ray powder diffraction data to contain tin in both the substitutional and interstitial octahedral sites of the corundum-related structure.

Synthesis and characterisation of tin-doped iron oxides

Tin-doped α-FeOOH (goethite) has been prepared directly by the hydrothermal processing of a precipitate. The morphology of the crystals, which is reflected in the 57Fe Mossbauer spectra, depends on the pH of the precipitating media. The 119Sn Mossbauer spectra are consistent with a complex microstructure around the octahedrally coordinated tin ions in the goethite structure and are sensitive to the changes in the magnetic structure of α-FeOOH resulting from tin doping. 57Fe Mossbauer spectroscopy and electron microscopy show acicular tin-doped α-FeOOH to be converted to tin-doped α-Fe2O3 with identical morphology by mild heating in air.

Preparation and characterisation of tin-doped α-FeOOH (goethite)

[1] With the prevalence of glass and molten silicates in volcanic environments, and the important role of surface emissivity in thermal infrared (TIR) measurements, it is imperative to characterize accurately the spectral features associated with silicate glasses and melts. A microfurnace has been developed specifically for use with a laboratory Fourier transform infrared (FTIR) spectrometer to collect the first in situ TIR emission spectra of actively melting and cooling silicate glasses. The construction, implementation, and calibration of the microfurnace spectrometer system are presented here. Initial testing of the microfurnace is also discussed, which includes acquisition of thermal emission spectra of a quartz powder (unmelted), a melted and cooled oligoclase feldspar, and glassy melt of rhyolitic composition. Unlike a solid material, which may only have bending and stretching vibrations within its molecular structure, a fully molten material will exhibit several more degrees of freedom in structural movement, thus changing its spectral character. Differences in spectral behavior and morphology are observed between a glass in a solid state and its molten counterpart, confirming previous field measurements of lower emissivity upon melting. This laboratory microfurnace system has been designed to quantify the TIR emission spectral behavior of glassy materials in various physical states. Ultimately, it is hoped that the microfurnace data will help improve the ability of field-based, airborne, and spaceborne TIR data to characterize glassy volcanic terranes.

/pdf/development-of-a-new-laboratory-technique-for-high-27txmwyh0v.pdf

Development of a new laboratory technique for high-temperature thermal emission spectroscopy of silicate melts

The 57Fe Mossbauer spectra recorded in situ from tin-doped Fe3O4 at elevated temperature in vacuo shows the Curie temperature to decrease with increasing concentrations of the dopant. Thermal treatment under oxidising conditions results in the initial formation of tin-doped γ-Fe2O3 which subsequently undergoes a phase transformation to tin-doped α-Fe2O3. 57Fe Mossbauer spectroscopy at elevated temperatures shows the Neel temperature for tin-doped γ-Fe2O3 to be lower than that of pure γ-Fe2O3. The 119Sn Mossbauer spectra recorded from all the tin-doped iron oxides show the presence of a hyperfine magnetic field at the Sn4+ site which is more complex in the spectra recorded from tin-doped γ-Fe2O3 and α-Fe2O3.

Mössbauer spectroscopic properties of tin-doped iron oxides

Mossbauer spectroscopy of basalt lava samples, exhibiting reversible thermal magnetization (Js-T) curves with Curie temperatures of about 840 K, has revealed considerable amounts of maghemite (γ-Fe2O3) in many samples. In view of the expected instability of maghemite at temperatures above 620 K, this reversibility came as a surprise. For further studies of the magnetization-temperature relationship of these minerals, we have constructed an elliptical radiation-heated furnace in which Mossbauer spectra can be acquired at temperatures between 300 and 900 K. Measurements at different temperatures have been obtained for two types of basalts, one in which the magnetic minerals are nearly pure magnetite and the other where the room temperature spectrum indicates a mixture of maghemite and magnetite. The two series show different features of the collapse of the internal magnetic hyperfine field, and the composition of minerals in the samples changes during the treatment, showing maghemite.

High temperature Mössbauer spectroscopy of titanomagnetite and maghemite in basalts

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High temperature Mössbauer spectroscopy of titanomagnetite and maghemite in basalts