Prereactional transformations in the topotactic conversion γ-FeOOH → γ-Fe2O3

Abstract

The transformation of the lepidocrocite (γ‐FeOOH) lattice into the maghemite (γ‐Fe2O3) spinel structure is studied by X‐ray analysis, infrared spectroscopy, and ferromagnetic resonance technique. It is found that the dehydroxylation connected with the thermal treatment of lepidocrocite yieldsan excess of Fe3+ ions in its lattice preceding the transformation into maghemite. The recorded type of ferromagnetic phase with lepidocrocite structure suggests arandom distribution of these Fe3+ ions in tetrahedral sites. This appears to be a pre‐condition for the topotactic conversion with the formation of γ‐Fe2O3 spinell lattice.

Factors Influencing the Texture and Stability of Maghemite Obtained from the Thermal Decomposition of Lepidocrocite

“…The thermal dehydration of lepidocrocite has merited the attention of many authors. 22,[26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] Wolska and Baszynsky, 41 Ghering et al, 42,43 and Bakker et al 22 have concluded from spectroscopic and magnetic studies that a molecular level prereactional dehydroxylation of the lepidocrocite begins at a temperature between 150 and 170 °C with the formation of superparamagnetic γ-Fe 2 O 3 nuclei, although the overall conversion of γ-FeOOH to γ-Fe 2 O 3 starts at about 200 °C. According to Naono et al 40 the acicular crystals of lepidocrocite undergo a topotactic dehydroxylation, where the original single crystal of lepidocrocite is replaced by a highly ordered aggregate of small maghemite crystals with a large amount of micropores.…”

“…The thermal dehydration of lepidocrocite has merited the attention of many authors. ,− Wolska and Baszynsky, Ghering et al, , and Bakker et al have concluded from spectroscopic and magnetic studies that a molecular level prereactional dehydroxylation of the lepidocrocite begins at a temperature between 150 and 170 °C with the formation of superparamagnetic γ-Fe 2 O 3 nuclei, although the overall conversion of γ-FeOOH to γ-Fe 2 O 3 starts at about 200 °C. According to Naono et al .…”