Thermal dehydration of goethite (α-FeOOH) at low temperature leads to complete transformation into a hematite-like phase before less than one half of the stoichiometric water content in goethite is expelled. The character of the remaining water has not bee explained as yet.
The variations of band positions in the infrared spectra and the specific changes in relative integrated intensities of X-ray lines with the increase of dehydration temperature suggested that OH-groups replace oxygen anions in the hematite crystal lattice, while the electroneutrality is preserved by the cation vacancies.
Comparison of integrated intensities observed with intensities calculated on this assumption fully confirms that the hematite-like phase is an iron deficient species with a general chemical formula α-Fe2−x/3-(OH)xO3−x.
The accurate measurements of lattice constants reveal that while the crystallographic co-axis decreases steadily with progressing thermal dehydroxylation, the ao-axis reaches a minimum at the temperature at which the nonuniform X-ray line broadening disappears. The largest unit cell of hematite phase after the goethite/hematite transition and the smallest one after the disappearance of selected broadening of X-ray diffraction peaks represent two transitional phases with hematite structure, protohematite and hydrohematite, displaying their own structural and compositional characteristics. The smallest unit cell appearing when the crystalline phase still holds about three percent of structurally bound water suggests that the stoichiometric hematite is thermodynamically less stable than this intermediate state.
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