The transformation of lepidocrocite during heating: A magnetic and spectroscopic study

Infrared (IR) spectroscopy, in combination with magnetic methods, was used to study the thermally induced transformation of synthetic lepidocrocite (γ-FeOOH) to maghemite (γ-Fe₂O₃). Magnetic analyses showed that the thermal conversion began at about 175°C with the formation of super-paramagnetic maghemite clusters. The overall structural transformation to ferrimagnetic γ-Fe₂O₃ occurred at 200°C and was complete around 300°C. At higher temperatures, the maghemite converted into hematite (α-Fe₂O₃). Observation of the transformation from γ-FeOOH to γ-Fe₂O₃ using variable-temperature IR spectroscopy indicated that dehydroxilation on a molecular level was initiated between 145°C and 155°C. The lag time between the onset of the breaking of OH bonds and the release of H₂O from lepidocrocite around 175°C can be explained by diffusive processes. Overall dehydroxilation and the subsequent breakdown of the lepidocrocite structure was complete below 219°C. The comparison of the magnetic and IR data provides evidence that the dehydroxilation may precede the structural conversion to maghemite.