Mesostructured iron oxyhydroxides. 2. Soft hydrothermal restructuring processes

The properties of mesostructured iron oxhydroxide-surfactant composites pre pared from weakly acidic (pH between 2.3 and 2.6) Fe(III) solutions by the addition of dodecyl sulfate are studied. The mesocomposites exhibit superpa ramagnetic behavior with the magnetic ordering within the inorganic part be ing antiferromagnetic. Mossbauer spectra indicate the presence of significa nt interparticle interactions, as deduced from the closeness of the blockin g temperature in these measurements and the peak temperature of zero-field- cooled magnetic susceptibility data. The blocking temperature in static mag netic measurements is dependent on the aging time of the mesostructured com posites in the mother liquor and varies between 17 K and around 30 K. This shift is attributed to an internal restructuring process within the inorgan ic part of the mesostructured composites, namely, further cross-linking bet ween not fully connected [Fe(O,OH)(6)] octahedra. This is supported by X-ra y absorption spectroscopy: Shortly aged samples, that is, 1 or 2 days, have an inorganic part resembling that of bulk lepidocrocite (gamma -FeOOH) wit h 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 goe thite and akaganeite. The increase in the blocking temperature with aging t ime can be interpreted in terms of growth of the magnetically active domain s.