Mesostructured iron oxyhydroxides. 1. Synthesis, local structure, and magnetism

The synthesis, local structure, and magnetism of lamellar iron(III) oxyhydr oxide-surfactant composites prepared by two different methods have been inv estigated in detail. In the first method, Fe(II) solutions are oxidized by H2O2 in the presence of CnH2n+1OSO3Na+ surfactants (n = 10, 12, 14, 16, 18) , leading to lamellar composites with an inorganic wall thickness of around 28 Angstrom. When a second method is used, namely, aging an Fe(III) soluti on for selected times after slightly increasing the pH with NH3 and subsequ ent addition of the surfactant, the inorganic wall thickness can be tuned b etween 19 and 26 Angstrom, employing the same surfactants. EXAFS analysis o f the Fe K edge X-ray absorption spectra reveals that the local structure o f the inorganic part is a reminder of those found for the bulk iron oxyhydr oxides goethite and akaganeite; that is, [Fe(O,OH)(6)] octahedra are predom inantly connected by common edges and corners, the ratio of edge to corner sharing being similar to the mentioned bulk oxyhydroxides. Whereas coordina tion numbers for the first oxygen coordination shell are around 6, confirmi ng an octahedral (or distorted octahedral) coordination around the Fe ions, coordination numbers found for the second and third Fe . . . Fe neighbors are low (around 2), indicating the presence of a considerable amount of vac ancies around the central absorber ion or, as an alternative description, a low degree of condensation of the oxyhydroxide. Complementary to the local structural picture given by EXAFS, Mossbauer spectra elucidate the inorgan ic iron oxyhydroxide walls to be built up by domains of different crystalli nity. The crystallinity is sensitive to the synthesis conditions used in th e preparation. For example, under aging in the presence of NH3, longer agin g times and higher temperatures result in a larger overall crystallinity of the inorganic part. By carefully controlling the reaction parameters, the thickness of the inorganic layers can be varied from around 19 Angstrom to around 30 Angstrom; also, the blocking temperatures of these superparamagne tic compounds observed by zero-field-cooled magnetization measurements can be controlled in the range between 4 and 30 K.