The Transformation of Lepidocrocite During Heating: A Magnetic and Spectroscopic Study

Abstract

Abstract--Infrared (IR) spectroscopy, in combination with magnetic methods, was used to study the thermally induced transformation of synthetic lepidocrocite (3,-FeOOH) to maghemite (~-Fe203). Magnetic analyses showed that the thermal conversion began at about 175"C with the formation of superparamagnetic maghemite clusters. The overall structural transformation to ferrimagnetic'y-Fe203 occurred at 200"C and was complete around 300"C. At higher temperatures, the maghemite converted into hematite (a-Fe203). Observation of the transformation from 3,-FeOOH to "y-Fe203 using variable-temperature IR spectroscopy indicated that dehydroxilation on a molecular level was initiated between 145"C and 155*C. The lag time between the onset of the breaking of OH bonds and the release of H20 from lepidocrocite around 1750C can be explained by diffusive processes. Overall dehydroxilation and the subsequent breakdown of the lepidocrocite structure was complete below 219*(3. The comparison of the magnetic and IR data provides evidence that the dehydroxilation may precede the structural conversion to maghemite.

Annealing Effects on the Metal and Semiconductor Nanowires Loaded Inside the Alumina Pores

“…The first one, associated with desorption of physisorbed molecular water, takes place even at T ann around 150 ºC, because the color and optical properties of alumina templates loaded with lepidocrocite nanowires evidently change under these conditions. This observation is in line with the indication (Gehring & Hofmeister, 1994) that, on the molecular level, the removal of OH groups in the lepidocrocite begins at 142-155 ºC. Based on the results obtained here by Mössbauer transmission spectroscopy and Raman scattering (not shown here) it seems likely that the second step, corresponding to the dehydroxylation of iron oxyhydroxide nanowires to γ-and a-Fe 2 O 3 , completely proceeds in the vicinity of 500 ºC although from literature data (Bernal et al 1957) the rhombic structure of synthetic lepidocrocite reconstructs to cubic maghemite in the 230-250 ºC region which at 375 ºC is transported completely to hematite with a hexagonal structure, e.g.…”

Treatment of Biogenic Iron-Containing Materials

“…41,43,45 Also, the studied materials demonstrate bands at 3450, 3000-2800, 1620, and 1380 cm -1 in the region of 600-4000 cm -1 . The bands at 3450 and 1620 cm -1 result from bending vibrations of hydrogen-bonded hydroxyl groups and physically adsorbed water on the surface of the samples, respectively.…”

“…Besides, the far-infrared spectrum shows that the treatment at temperatures up to 250 °С has resulted in bands registered at about 326, 457, and 538 cm -1 (Figure 4), which are characteristic of metal-oxygen vibrations in Fе 2 O 3 . 43 Thus, the thermally induced spectral changes indicate a phase α-FeOOH·хН 2 О → Fе 2 O 3 transformation. It is worth to note that the bands at 2850, 2922, and 2962 cm -1 were not changed after dehydration.…”

Low‐temperature magnetic properties of lepidocrocite

“…[6] The synthesis of sample L89 is described by Gehring and Hofmeister [1994] and sample P23 is defined by Murad and Schwertmann [1984]. The purity of both samples was verified again for this study by X-ray powder diffractometry (XRD) with a Bragg-Brentano camera (SCINTAG XDS 2000, Cu-K a radiation, solid-state detector).…”

“…Two well-characterized samples, previously described in the literature, were used [Gehring and Hofmeister, 1994;Murad and Schwertmann, 1984]. Gehring and Hofmeister [1994] examined the thermal conversion of lepidocrocite (L89) to maghemite and hematite during heating. Sample P23 was used in the original Mössbauer study of Murad and Schwertmann [1984] and again in the study of De Grave et al [1986].…”