Microstructural properties of Fe-oxide powders obtained by precipitation from FeCl3 solutions

The precipitation of Fe-oxides was performed by adding NaOH or urotropin to FeCl3 solutions and aging the precipitation systems at 90 degrees C. Phase composition and microstructural properties of Fe-oxide powders were investigated using Fourier transform infrared spectroscopy, Mossbauer spectroscopy, X-ray diffraction and transmission electron microscopy. beta-FeOOH was obtained in all precipitation systems with final pH values up to 1.80. However, when the precipitation was for a long time at pH less than or equal to 1.80 the appearance of alpha-Fe2O3 phase was also observed. In the presence of urotropin the size of submicrometer beta-FeOOH rods increased with time of aging thus forming a fibril structure of near micrometer size. The crystallite size of beta-FeOOH varied between 7 and 11 nm and this indicated that beta-FeOOH fibrils possessed a substructure. After 30 days of aging, the presence of a alpha-Fe2O3 phase was also detected by XRD. alpha-Fe2O3 was obtained as a single phase by precipitation with NaOH at pH 4.20-4.40 and aging of the precipitation systems up to 3 days. When the pH of precipitation was increased to 6.00-6.50 the formation of alpha-FeOOH particles, in addition to alpha-Fe2O3, was observed. TEM showed that the size and morphology of particles strongly depended on the precipitation conditions. The mechanism of the formation of Fe-oxide phases is discussed. (C) 1998 Elsevier Science S.A. All rights reserved.