Factors influencing the texture and stability of maghemite obtained from the thermal decomposition of lepidocrocite

The thermal decomposition of a series of lepidocrocite samples has been monitored by means of the Constant Rate Thermal Analysis (CRTA). This method allows controlling the reaction temperature in such a way that both the reaction rate and the partial pressure of the water vapor generated in the reaction are maintained constant at values that call be arbitrarily selected by the user. The final products obtained from the thermal dehydroxylalion of the lepidocrocite precursors under residual pressures of water vapor ranging from 5 x 10(-5) mbar to 10 mbar have been characterized by TEM, XRD, and Mosbauer spectroscopy. It has been concluded that they preserve the shape and size of the corresponding precursors. The precise control of the partial pressure of the water vapor self-generated in the above reaction allows tailoring the internal porosity of the maghemite-hematite mixture obtained as final product. It has been shown that the larger the particle size of the starting lepidocrocite is, the larger the porosity of the iron oxide obtained. The ratio gamma-Fe2O3/alpha-Fe2O3 is strongly dependent on both the texture of the starting lepidocrocite and the partial pressure of water vapor selected for performing its dehydration. Thus, the percentage! of maghemite increases by decreasing the particle size of the stal ting lepidocrocite and by increasing the residual pressure of water vapor. An interpretation of this behavior is given.