Magnetite particles, Mossbauer spectra

Natural magnetite may also occur with a small particle morphology yielding a Mossbauer spectrum with asymmetrically shaped lines. Due to the increased overlap of the lines of both sextets in that case, it becomes difficult to determine the ratio R = S(Fe )/S(Fe ) accurately. 

Flo. 37. Mossbauer spectra of small Fe304 particles exposed to air at room temperature and after C02/C0 treatment at 700 K. (a) and (b) are room temperature Mossbauer spectra of nonsupported magnetites exposed to air for 80 days (c) is sample (b) treated at 700 K with a C02/C0 mixture. Spectrum at room temperature. Zero velocity is with respect to a 57Co in copper source. Figure according to Tops0c el al. (237). 

In another study, the effect of silica incorporation into the Fe O lattice was studied (5,49,50) A 20% Fe O on silica catalyst was prepared using conventional techniques. Ir was found that while direct oxidation of the catalyst at 800 K produced the expected a -catalyst was previously reduced in CO/CO2 to produce magnetite, then subsequent oxidation resulted in the formation of y-Fe203 Figure 10 shows MOssbauer spectra of this catalyst after various thermal treatments. In these spectra, the central doublets were demonstrated to be a result of small iron oxide particles which were superparamagnetic at the conditions where the spectrum was recorded. The suppression of the y - oOg to a -Fe203 transition is characteristic of the substitution of roreign cations into the mag-... 

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