57Fe Mossbauer, FT-IR and TEM investigations of Fe-oxide powders obtained from concentrated FeCl3 solutions

The chemical and microstructural properties of oxide phases precipitated from concentrated FeCl3 solutions were investigated using Fe-57 Mossbauer spectroscopy, Fourier transform infrared spectroscopy and transmission electron microscopy. The initial pH of the precipitation system (acidic or highly alkaline medium) determined the mechanism of phase transformations. In a highly alkaline medium two competitive mechanisms exist: (a) the nucleation and crystal growth of alpha-Fe2O3 inside the condensed Fe(OH)(3) gel and (b) the crystallization of alpha-FeOOH which transforms to alpha-Fe2O3 by a dissolution/reprecipitation mechanism. In the present work, spectroscopic results indicated that before a-FeOOH was detectable (or formed) at 60 degrees C, superparamagnetic and/or amorphous Fe-oxide particles (probably alpha-Fe2O3) were formed. Once alpha-FeOOH was formed, its fraction in the precipitate increased rapidly. In an acidic medium (partially neutralized FeCl3 solution) alpha-Fe2O3, was formed from beta-FeOOH by a dissolution/reprecipitation mechanism. The complete interpretations of Fe-57 Mossbauer and FT-IR spectra were given. TEM observation showed that the particle size and morphology of the oxide phases were very dependent on the experimental parameters. (C) 1998 Elsevier Science S.A. All rights reserved.