Mesostructured iron oxyhydroxides. 2. Soft hydrothermal restructuring processes

The properties of mesostructured iron oxhydroxide - surfactant composites prepared from weakly acidic (pH between 2.3 and 2.6) Fe(III) solutions by the addition of dodecyl sulfate are studied. The mesocomposites exhibit superparamagnetic behavior with the magnetic ordering within the inorganic part being antiferromagnetic. Mössbauer spectra indicate the presence of significant interparticle interactions, as deduced from the closeness of the blocking temperature in these measurements and the peak temperature of zero-field-cooled magnetic susceptibility data. The blocking temperature in static magnetic measurements is dependent on the aging time of the mesostructured composites in the mother liquor and varies between 17 K and around 30 K. This shift is attributed to an internal restructuring process within the inorganic part of the mesostructured composites, namely, further cross-linking between not fully connected [Fe(O,OH)₆] octahedra. This is supported by X-ray absorption spectroscopy: Shortly aged samples, that is, 1 or 2 days, have an inorganic part resembling that of bulk lepidocrocite (γ-FeOOH) with dominantly corner-linked [Fe(O,OH)₆] units. With increasing aging time, further cross-linking within the inorganic part results in an increase of edge-sharing octahedra, making the local structures similar to those of goethite and akaganéite. The increase in the blocking temperature with aging time can be interpreted in terms of growth of the magnetically active domains.