The effects of the Cr-dopant on the phenomenology of the precipitation of acicular α-FeOOH particles, the formation of solid solutions, particle size and shape were investigated using X-ray powder diffraction (XRD), Mössbauer and Fourier transform infrared (FT-IR) spectroscopies and field emission scanning electron microscopy (FE-SEM). Acicular and monodisperse α-FeOOH particles were precipitated at a very high pH by heating the suspension obtained by adding a tetramethylammonium hydroxide solution to the aqueous solution of FeCl₃. The influence of the Cr-dopant was investigated by adding varying amounts of Cr³⁺ ions to the initial FeCl₃ solution, where r = 100·[Cr]/([Cr]+[Fe]) is the added amount of chromium. An XRD analysis of the obtained powders (with r values from 0 to 23.08) showed only the presence of the diffraction lines characteristic of α-FeOOH. Mössbauer spectroscopy showed a decrease in hyperfine magnetic field values of α-FeOOH sextets with an increase in Cr-addition, which indicates Cr incorporation into the α-FeOOH structure. OH bending bands in the FT-IR spectra showed only a slight change in position with an increased r, but a considerable increase in the lattice band wave number indicated a decreased thickness of α-FeOOH particles. This conclusion was confirmed by FE-SEM observations.

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