Influence of aging in an alkaline medium on the microstructural properties of α-FeOOH

The influence of the aging in a highly alkaline medium at 160 °C on the microstructural properties of α-FeOOH was monitored by XRD, 57Fe Mössbauer and FT-IR spectroscopies and FE-SEM. In the synthesis of acicular and monodisperse α-FeOOH particles, tetramethylammonium hydroxide was used as a precipitating agent. The crystal growth of α-FeOOH by the mechanism of lateral arraying was observed with prolonged aging time. Smaller α-FeOOH particles were dissolving with time, thus providing Fe(OH)4- ions for the additional growth of α-FeOOH particles and improvement of their crystal shape. The XRD analysis confirmed that the growth of α-FeOOH crystallites perpendicularly to the crystallographic c-axis was favoured. The Mössbauer spectrum of α-FeOOH showed a distribution of the hyperfine magnetic field (HMF) which gradually narrowed with a longer aging time. The average value of the HMF distribution was shifted from 35.3 to 37.1 T between 2 and 168 h of aging. Characteristic IR bands of α-FeOOH were sensitive to the microstructural changes of α-FeOOH particles during their aging in the suspension in a highly alkaline medium at 160 °C.