57Fe Mössbauer, FT-IR and TEM Observations of Oxide Phases Precipitated from Concentrated Fe(NO3)3 Solutions

Concentrated Fe(NO3)3 Solutions (final Fe3+ concentration of 1 M) were completely or partially neutralized by addition of concentrated NaOH solution, and then aged at various temperatures and times. The oxide phases were investigated using Mössbauer spectroscopy, FT-IR spectroscopy and transmission electron microscopy. In a highly alkaline medium, single phase α-FeO(OH) was obtained by the aging of Fe(OH)3 suspension at room temperature (RT) for 135 days. The Mössbauer spectrum of this sample at 298 K showed superposition of the Central quadrupole doublet and a collapsing sextet, while at 80 K a sextet corresponding to α-FeO(OH) was recorded. In the highly alkaline medium, an amorphous material was obtained after heating the Fe(OH)3 suspension at 333 K for 1 day. Spectroscopic investigations provided a certain evidence of the presence of α-Fe2O3 in this material. With prolonged time of heating, α-FeO(OH) appeared at 333 K and its fraction showed a tendency to increase. It was suggested that, in a highly alkaline medium, nucleation and crystallization of α-Fe2O3 occurred inside the amorphous gel, while α-FeOOH crystallized by a dissolution/reprecipitation mechanism. After neutralization of 90% of 1 M Fe(NO3)3 solution, lemon-shaped -Fe2O3 particles were produced at 398 K between 6 and 24 h. Interpretations of the spectroscopic and electron microscopic measurements are given