A study of bernalite, Fe(OH) ₃ , using Mössbauer spectroscopy, optical spectroscopy and transmission electron microscopy

To study the crystal chemistry of bernalite, Fe(OH) ₃ , and the nature of the octahedral Fe ³⁺ environment, Mössbauer spectra were recorded from 80 to 350 K, optical spectra were recorded at room temperature and a sample was studied using transmission electron microscopy. The Mössbauer spectrum of bernalite consists of a single six-line magnetic spectrum at 80 K. A broadened six-line magnetic spectrum with significantly less intensity is observed at higher temperatures, and is attributed to a small fraction of bernalite occurring as small particles. The variation of hyperfine magnetic field data for bulk bernalite with temperature is well described by the Weiss molecular field model with parameters of H ₀ = 55.7±0.3 T and T _N = 427±5K. The centre shift data were fitted to the Debye model with parameters δ ₀ =0.482±0.005 mm/s (relative to α-Fe) and Θ _M =492±30 K. The quadrupole shift is near zero at 300 K, and does not vary significantly with temperature. Absorption spectra in the visible and near infrared range show three crystal field bands of Fe ³⁺ at 11 300, 16000 and 23 200 cm ^-1 , giving a crystal field splitting of 14 570 cm ^-1 and Racah parameters of B=629 cm ^-1 and C=3381 cm ^-1 . Infrared reflection spectra show two distinct OH-stretching frequencies, which could correspond to two structurally different types of OH groups. A band was also observed at 2250 cm ^-1 , suggesting the presence of molecular CO ₂ in the large cation site. Analytical transmission electron microscopy indicates that Si occurs within the bernalite structure as well as along domain boundaries. Electron diffraction and imaging show that bernalite is polysynthetically twinned along {100} planes with twin domains ranging from 3 to 20 nm in thickness. Results are discussed with respect to the nature of the octahedral Fe ³⁺ site, and compared with values for other iron oxides and hydroxides.