MECHANISM AND REACTION-KINETICS IN THE SOLID-PHASE TRANSFORMATION ALPHA-FEOOH-]ALPHA-FE2O3 STUDIED BY MOSSBAUER-SPECTROSCOPY

The thermal decomposition of alpha-FeOOH, prepared by air oxidation of Fe(OH)(2) at room temperature, was investigated by means of Mossbauer spectroscopy and transmission electron microscopy. The measurements w ere performed on powder samples previously annealed between 200 and 70 0 degrees C. From Mossbauer spectral data the reaction isotherms, rate constants and the activation energy of the process were determined. T he transformation into alpha-iron oxide takes place preserving the nee dle-like shape of particles. An Avrami-Erofeev mechanism was identifie d to characterise the solid phase transformation of alpha oxyhydroxide to alpha iron oxide.