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

The synthesis, local structure, and magnetism of lamellar iron(III) oxyhydroxide-surfactant composites prepared by two different methods have been investigated in detail. In the first method, Fe(II) solutions are oxidized by H(2)O(2) in the presence of C(n)H(2n+1)OSO(3)Na(+) 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) solution for selected times after slightly increasing the pH with NH(3) and subsequent addition of the surfactant, the inorganic wall thickness can be tuned between 19 and 26 Angstrom, employing the same surfactants. EXAFS analysis of the Fe K edge X-ray absorption spectra reveals that the local structure of the inorganic part is a reminder of those found for the bulk iron oxyhydroxides goethite and akaganeite; that is, [Fe(O,OH)(6)] octahedra are predominantly connected by common edges and corners, the ratio of edge to corner sharing being similar to the mentioned bulk oxyhydroxides. Whereas coordination numbers for the first oxygen coordination shell are around 6, confirming 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 vacancies 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 inorganic iron oxyhydroxide walls to be built up by domains of different crystallinity. The crystallinity is sensitive to the synthesis conditions used in the preparation. For example, under aging in the presence of NH3, longer aging 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 superparamagnetic compounds observed by zero-field-cooled magnetization measurements can be controlled in the range between 4 and 30 K.