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 lepido crocite (gamma-FeOOH) to maghemite (gamma-Fe2O3). Magnetic analyses sh owed that the thermal conversion began at about 175 degrees C with the formation of superparamagnetic maghemite clusters. The overall struct ural transformation to ferrimagnetic gamma-Fe2O3 occurred at 200 degre es C and was complete around 300 degrees C. At higher temperatures, th e maghemite converted into hematite (alpha-Fe2O3). Observation of the transformation from gamma-FeOOH to gamma-Fe2O3 using variable-temperat ure IR spectroscopy indicated that dehydroxilation on a molecular leve l 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 H2O f rom lepidocrocite around 175 degrees C can be explained by diffusive p rocesses. Overall dehydroxilation and the subsequent breakdown of the lepidocrocite structure was complete below 219 degrees C. The comparis on of the magnetic and IR data provides evidence that the dehydroxilat ion may precede the structural conversion to maghemite.