The phase analysis and nanostructural properties of the precipitates formed by forced hydrolysis of FeCl3 solutions at 160 degrees C in the presence of Cr3+ ions and hexamethylenetetramine (HMTA) were investigated. The phase analyses of the precipitates showed the presence of ferrihydrite (amorphous-like), alpha-FeOOH, beta-FeOOH, and alpha-Fe2O3. The exact phase composition was strongly dependent on the starting chemical composition in the solution and the precipitation time. Ferrihydrite, alpha-FeOOH, and beta-FeOOH were the precursors of alpha-Fe2O3 as the end-product. Upon 2 h of autoclaving at 160 degrees C, the precipitates consisted of nanoparticles similar to 35 to similar to 50 nm in size, whereas after 24 h their size increased to similar to 100 nm. New shapes of these nanoparticles were found. In these precipitation systems crystalline chromium (hydrous)oxide phase was not detected. However, the formation of solid solutions between Cr3+ ions and iron oxides could not be excluded taking into account Energy-dispersive X-ray spectroscopy (EDS) measurements.
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