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Iodine doping of polyferrocenylenesilylene(germylene) thin films, [(η 5 -R 2 EC 5 H 4 )Fe(η 5 -C 5 H 4 )] n , E ) Si, Ge, transforms them from insulators to conducting materials. The process was monitored by IR, Raman, and UV/vis spectroscopy and occurs via irreversible electron transfer to form of ferrocenium ion centers with [I 3 ]counteranions. The latter further react with I 2 to form [I 5 ]polyiodide anions in a reversible manner. A Mo ¨ssbauer spectrum of the doped polymer suggests that alternate Fe centers have been oxidized. Treatment of the doped films with NH 3 results in a typical compensation process. Experimental SectionGeneral Procedures. The various polymeric materials were synthesized using published routes. 1,2 Doping was performed, in vacuo, by exposing thin films of the polymers, spincoated or cast from benzene solutions (500-6000 Å on quartz, KBr or CaF 2 substrates), to iodine vapor. Conductivity measurements were obtained using published techniques on these samples, 3a and UV/vis spectra were recorded concurrently. Doping was continued until constant conductivity values and constant electronic spectra were obtained. In all cases the constancy of both occurred at the same level of doping. UV/ vis spectra were recorded on Perkin-Elmer λ-14 or Carl Zeiss M-40 spectrometers, Raman spectra on a Jobin-Yvon T64000

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.