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°C. Phase composition and microstructural properties of Fe–oxide powders were investigated using Fourier transform infrared spectroscopy, Mössbauer spectroscopy, X-ray diffraction and transmission electron microscopy. β-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≤1.80 the appearance of α-Fe2O3 phase was also observed. In the presence of urotropin the size of submicrometer β-FeOOH rods increased with time of aging thus forming a fibril structure of near micrometer size. The crystallite size of β-FeOOH varied between 7 and 11 nm and this indicated that β-FeOOH fibrils possessed a substructure. After 30 days of aging, the presence of a α-Fe2O3 phase was also detected by XRD. α-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 α-FeOOH particles, in addition to α-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.