High-purity Fe3S4 (greigite) microcrystals with octahedral shape were synthesized via a simple hydrothermal method using a surfactant. The as-prepared samples have the inverse spinel structure with high crystallinity. The saturation magnetization (M s) reaches 3.74 μB at 5 K and 3.51 μB at room temperature, which is larger than all reported values thus far. Electrical transport measurements show metallic behavior with a resistivity 40 times lower than in any previous report. The potential use of greigite as an anode in lithium-ion batteries was investigated by cyclic voltammery and galvanostatic discharge–charge cycling on as-prepared samples. The discharge capacity was 1161 mAh/g in the first cycle and 563 mAh/g in the 100th cycle. This excellent electrochemical performance can be attributed to the high purity, crystallinity, and favorable morphology of the products.

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FeSe exists as a hexagonal NiAs-like crystal structure with a large number of ordered intrinsic vacancies. It is an ideal candidate for studying the effect of defects on properties such as magnetism and electrical transport. In this work, highly crystalline FeSe with the 3c crystal structure was synthesized by a solid-state reaction. Sharp changes in the magnetization at 100 K confirm a rotation of the spins from the ab plane to the c axis with decreasing temperature. We observe an interesting metal-insulator transition at the same temperature as the spin-direction changes. We propose that locked spins in the grain boundaries induce electron localization and result in the metal-insulator transition. Electron localization is confirmed by X-ray photoelectron spectroscopy of the Fe 2p peaks, which exhibit two characteristic satellite peaks. This mechanism is also verified by comparing it with the properties of the 4c-FeSe crystal structure.

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The green alga, Chlamydomonas reinhardtii, can photoproduce molecular H 2 via ferredoxin and the reversible [Fe]hydrogenase enzyme under anaerobic conditions. Recently, a novel approach for sustained H 2 gas photoproduction was discovered in cell cultures subjected to S-deprived conditions (A. Melis, L. Zhang, M. Forestier, M.L. Ghirardi, M. Seibert [2000] Plant Physiol 122: 127-135). The close relationship between S and Fe in the H 2 -production process is of interest because Fe-S clusters are constituents of both ferredoxin and hydrogenase. In this study, we used Mö ssbauer spectroscopy to examine both the uptake of Fe by the alga at different CO 2 concentrations during growth and the influence of anaerobiosis on the accumulation of Fe. Algal cells grown in media with 57 Fe(III) at elevated (3%, v/v) CO 2 concentration exhibit elevated levels of Fe and have two comparable pools of the ion: (a) Fe(III) with Mö ssbauer parameters of quadrupole splitting ϭ 0.65 mm s Ϫ1 and isomeric shift ϭ 0.46 mm s Ϫ1 and (b) Fe(II) with quadrupole splitting ϭ 3.1 mm s Ϫ1 and isomeric shift ϭ 1.36 mm s Ϫ1 . Disruption of the cells and use of the specific Fe chelator, bathophenanthroline, have demonstrated that the Fe(II) pool is located inside the cell. The amount of Fe(III) in the cells increases with the age of the algal culture, whereas the amount of Fe(II) remains constant on a chlorophyll basis. Growing the algae under atmospheric CO 2 (limiting) conditions, compared with 3% (v/v) CO 2 , resulted in a decrease in the intracellular Fe(II) content by a factor of 3. Incubating C. reinhardtii cells, grown at atmospheric CO 2 for 3 h in the dark under anaerobic conditions, not only induced hydrogenase activity but also increased the Fe(II) content in the cells up to the saturation level observed in cells grown aerobically at high CO 2 . This result is novel and suggests a correlation between the amount of Fe(II) cations stored in the cells, the CO 2 concentration, and anaerobiosis. A comparison of Fe-uptake results with a cyanobacterium, yeast, and algae suggests that the intracellular Fe(II) pool in C. reinhardtii may reside in the cell vacuole.Light energy conversion by algae, higher plants, and cyanobacteria is accompanied by water oxidation on the donor side of photosystem II (PSII) with the resultant evolution of molecular O 2 . The electrons extracted from water by PSII are transported to ferredoxin and NAPD ϩ via photosystem I (PSI), where they are normally used to fix CO 2 . However, after anaerobic incubation in the dark, illumination of Chlamydomonas reinhardtii (Greenbaum, 1982), Chlorella fusca (Kessler, 1974), Scenedesmus obliquus (Gaffron and Rubin, 1942), and some other species of algae leads to the expression of H 2 -evolution function. Molecular H 2 is produced as a result of ferredoxin-mediated electron transport to an induced, reversible [Fe]hydrogenase (rather than to NAPD ϩ and the Benson-Calvin Cycle) where the enzyme catalyzes the reduction of protons to H 2 gas.There are several types of hydrogenases tha...