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Magnetic properties of ultrafine clusters of Fe HO ·4HO (ferrihydrite, FH), isolated in pores of polysorb, were studied by Mossbauer spectroscopy and by thermodynamical analysis. Thermodynamical analysis allowed the conclusion that magnetic properties of ultrafine clusters cannot be interpreted in terms of a secondorder magnetic phase transition or of superparamagnetic behavior alone but require the consideration of a jumplike first order magnetic phase transition (JMT). The critical radius R below which the JMT is to be expected in clusters was derived from thermodynamic criteria. It was determined as R "2 /(1!¹ /¹ ), where , and are constants derived from surface energy, magnetostriction, compressibility and ¹ "3/2 Nk ¹ (N is the number of iron atoms, k is the Boltzmann constant, ¹ is the Curie temperature of the clusters). For the smallest FH clusters isolated in pores of polysorb, the critical radius and the JMT temperature were estimated by Mossbauer spectroscopy to be R :1.5-2.0 nm and ¹ (+2 :4.2-6 K, respectively. Satisfactory agreement between the value R , estimated from the experimental data and the one derived by thermodynamical analysis was achieved. Interfacial (cluster-surface) and intercluster interactions were found to destroy the JMT effect and to give rise to a second-order magnetic phase transition.

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Structural and thermal characteristics of an amorphous iodine complex of a ferrocenylenesilylene polymer, [-(C 5 H 4 )Fe(C 5 H 4 )Si(CH 3 )(C 6 H 5 )‚(I 1.5 )-] n , have been measured using 57 Fe Mo ¨ssbauer, IR and Raman spectroscopy, X-ray diffraction, and thermomechanical analysis. The iodine-doped macromolecules are mixed-valence salts with a localized electron structure on the 57 Fe Mo ¨ssbauer time scale, in which ferrocenylene units and ferrocenium moieties coexist. These macromolecules consist of blocks containing ferrocenium moieties and sequences of nonoxidized ferrocenylene units. The environment about the ferrocenium moieties consists of both single I 3and polyiodide [I 3 ] -‚nI 2 species. Mo ¨ssbauer data, collected in the range 5-300 K, are interpreted in terms of the temperature dependence of the Lamb-Mo ¨ssbauer factor and reversible electron transfer with an activation energy of 4.7 kJ/mol. The transformations of [I 3 ] -+ I 2 clusters to I 3are proposed to accompany Fe(II)-Fe(III) oxidation.

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