Iron chalcogenides

Iron sulfide as pyrite (FeS2) has been shown to be a promising photoactive material for photoelectrochemical and photovoltaic solar cells. Whereas a variety of methods have been employed for the preparation of thin films of this material, including CVT, MOCVD, spray-pyrolysis, and sulfidation of either iron oxide or iron, the direct efectrodeposition of FeS2 thin films has proven to be problematic. 

The potentiostatic electrodeposition of iron selenide thin films has been reported recently in aqueous baths of ferric chloride (FeCb) and Se02 onto stainless steel and fluorine-doped TO-glass substrates [193], The films were characterized as polycrystalline and rich in iron, containing in particular a monoclinic FesSea phase. Optical absorption studies showed the presence of direct transition with band gap energy of 1.23 eV. 

Preliminary results on the SrFe, Cr cOa y phase suggest that compositions with X = 0-20 and 0-30 are cubic perovskites with Fe and Fe cations magnetically ordered [184], The spectrum lines are very broad in the magnetic region. When x = 0-5 only Fe ions are present. 

There are comparatively few data available for iron sulphides and other chalcogenides, though interesting comparisons can be made in those cases where there are equivalent oxide systems. The same broad trends in the Mossbauer systematics are observed. Thus for chemical isomer shifts 

Further, when direct comparison between oxides and chalcogenides can be made the chemical isomer shifts and magnetic hyperfine fields observed for the chalcogenides are less than those in the corresponding oxide phases because of covalency effects. 

Stoichiometric FeS has the NiAs structure. It is antiferromagnetic at room temperature [185-188], with a magnetic field of 309 kG. The quadrupole interaction e qQ is probably about —0-85 mm s with the spin and eq axes angled at 48° to each other [187]. It was believed to undergo a change in the direction of the magnetic axis from along the c axis at low temperature 

Both the chemical isomer shift and the quadrupole splitting in FeSa show appreciable pressure dependence [193]. 

---

Oxides and other chalcogenides Table 25.4 Electronic spin-states of iron... 

Unlike the metal atoms in the chalcogenides discussed in the previous section, the iron atoms in iron(II) tetraphenyl porphyrins and the related iron(II) phthalocyanine generally occupy crystallographic sites with symmetry T or 1 only. The exception is the iron site in iron(II) tetraphenyl porphyrin (FeTPP), space group I42d, which has 4 symmetry. 

The pyrite (FeS2) structure (C2) consists of molecular ions (Fig. 1.8). The structure is closely related to NaCl from which it may be derived by replacing Na by Fe and Cl by S , with the centre of the S ion occupying the chloride position. Each iron is octahedrally coordinated but each sulphur is tetrahedrally surrounded (one S and three Fe). Several transition metal chalcogenides crystallize in this structure. 

Heterometal alkoxide precursors, for ceramics, 12, 60-61 Heterometal chalcogenides, synthesis, 12, 62 Heterometal cubanes, as metal-organic precursor, 12, 39 Heterometallic alkenes, with platinum, 8, 639 Heterometallic alkynes, with platinum, models, 8, 650 Heterometallic clusters as heterogeneous catalyst precursors, 12, 767 in homogeneous catalysis, 12, 761 with Ni—M and Ni-C cr-bonded complexes, 8, 115 Heterometallic complexes with arene chromium carbonyls, 5, 259 bridged chromium isonitriles, 5, 274 with cyclopentadienyl hydride niobium moieties, 5, 72 with ruthenium—osmium, overview, 6, 1045—1116 with tungsten carbonyls, 5, 702 Heterometallic dimers, palladium complexes, 8, 210 Heterometallic iron-containing compounds cluster compounds, 6, 331 dinuclear compounds, 6, 319 overview, 6, 319-352... 

Numerous transition metals ions form cluster complexes with chalcogenide anions [42-52], Iron and sulphur are unique elements in the sense that no two other elements can generate such a large diversity of cluster structures. This is the consequence of two stable oxidation states of iron ions and strong Fe-S bonds of significantly covalent character [53], Moreover, numerous structures are stable in several oxidation states, so these clusters serve as electron reservoirs in biological systems [51], This is why iron-sulphur proteins usually catalyze redox reactions. 

Because intermetallic systems undoubtedly display certain special features that follow from their metallic binding forces, considerable importance attached to the growing evidence that the chalcogenides, the essentially ionic oxides, the nitrides, and other representative binary compounds of the transition metals were, not infrequently, both variable and irrational in composition. Schenck and Ding-mann s equilibrium study of the iron-oxygen system (39) was notable in this connection They showed that stoichiometric ferrous oxide, FeOi 000, the oxide of an important and typical valence state, did not exist. It lay outside the broad existence field of a nonstoichiometric phase. It is, perhaps, still not certain... 

Chalcogenides Solid-state Chemistry Copper Enzymes in Denitrification Copper Hemocyanin/Tyrosinase Models Copper Proteins Oxidases Copper Proteins with Dinuclear Active Sites Copper Proteins with Type 1 Sites Copper Proteins with Type 2 Sites Iron Sulfitf Models of Protein Active Sites Iron-Snlfiir Proteins Nickel Enzymes Cofactors Nickel Models of Protein Active Sites Polynuclear Organometallic Cluster Complexes. 

As with Ti02, CdS can be used to photocatalyse reactions other than water cleavage. Oxidation of halide ions " proceeds smoothly at chalcogenide electrodes and n-type CdS can be used to photo-oxidize NO in the presence of iron(n) complexes. Similar studies have described the photoassisted reduction of CO2 to CO and the photo-oxidation of formic acid, formaldehyde, and methanol. ... 

Ternary compounds (not alloys) such as CdCr2Sc4 can be grown from a PbCl2-CdCl2 flux. These are magnetic semiconductors, but they have been eclipsed by the manganese- and iron-based alloys with zinc and cadmium chalcogenides, such as Cdi -,cMn,tTe. The pure compounds, usually made by direct reaction, are often used as... 

Scotford DM (1975) A test of aluminum in quartz as a geothermometer. Am Mineral 60 139-142 Scott HG (1975) Phase relationship in the zirconia-yttria system. J Mater Sci 10 1827-1835 Seifert F, Czank M, Simons B, Schmahl W (1987) A conunensurate-inconunensurate phase transition in iron-bearing ermanites. Phys Chem Minerals 14 26-35 Shannon RD (1976) Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr A32 751-767... 

---