Relation of Infrared, Crystallochemical, and Morphological Properties of Al-Substituted Hematites

Synthetic Al-hematites prepared from ferrihydrites, at low (∼ 100°C) and high (400° and 800°C) temperatures were studied for their morphological, crystallochemical, and infrared (IR) characteristics. Low-temperature Al-hematites had a platy morphology (the plate thickness was inversely related to amount of Al substitution), and the high temperature Al-hematites showed a poorly defined morphology due to interparticle sintering. In the low-temperature Al-hematites shifts in the IR mode frequencies were noted and could be partly explained by a shape factor that was deduced from particle morphology. The intrinsic effect of Al substitution, however, was to produce shifts of as much as 10–15 cm−1 for the highest Al substitution (∼ 16%). Similar shifts were observed for the high-temperature hematites in which morphology was not appreciably affected by Al substitution.