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 (gamma -FeOOH) to maghemite (gamma -Fe (sub 2) O (sub 3) ). Magnetic analyses showed that the thermal conversion began at about 175 degrees C with the formation of super-paramagnetic maghemite clusters. The overall structural transformation to ferrimagnetic gamma -Fe (sub 2) O (sub 3) occurred at 200 degrees C and was complete around 300 degrees C. At higher temperatures, the maghemite converted into hematite (alpha -Fe (sub 2) O (sub 3) ). Observation of the transformation from gamma -FeOOH to gamma -Fe (sub 2) O (sub 3) using variable-temperature IR spectroscopy indicated that dehydroxilation on a molecular level was initiated between 145 degrees C and 155 degrees C. The lag time between the onset of the breaking of OH bonds and the release of H (sub 2) O from lepidocrocite around 175 degrees C can be explained by diffusive processes. Overall dehydroxilation and the subsequent breakdown of the lepidocrocite structure was complete below 219 degrees C. The comparison of the magnetic and IR data provides evidence that the dehydroxilation may precede the structural conversion to maghemite.