Iron oxide, crystal defects

It has been known for a long time that two ciystalline solids, one which of starts at the surface of the other, often show mutual orientation relationships this is the process of epitaxy. Studies such as thin layers of oxides formed by oxidation of a metal, for example, have shown such a process. Figure 11.6 shows the example of the schematic orientations of cubic iron oxide (FeO) formed on the 001 side of the cubic iron. The side of the iron oxide cube is 2.86 A. The parameter of FeO is 4.29 A. In the isolated crystal, FeO iron atoms are spaced at 4.29 / VI = 3.89 A intervals, resulting in a slight deformation of the iron oxide crystal. If the layer of FeO is thick, the defect fades away from the interface the oxide has found its own structure. 

In contrast, the reddish-brown jerrihydrite (often wrongly termed amorphous iron oxide or hydrous ferric oxide (HFO) ) is widespread in surface environments. It was first described by Chukhrov et al. in 1973. Unlike the other iron oxides it exists exclusively as nano-crystals and unless stabilized in some way, transforms with time into the more stable iron oxides. Ferrihydrite is, thus, an important precursor of more stable and better crystalline Fe oxides. Structurally ferrihydrite consists of hep anions and is a mixture of defect-free, and defective structural units.The composition, especially with respect to OH and H2O, seems to be variable. A preliminary formula, often used, is FesOgH H2O. 

The solubility and the hydrolysis constants enable the concentration of iron that will be in equilibrium with an iron oxide to be calculated. This value may be underestimated if solubility is enhanced by other processes such as complexation and reduction. Solubility is also influenced by ionic strength, temperature, particle size and by crystal defects in the oxide. In alkaline media, the solubility of Fe oxides increases with rising temperature, whereas in acidic media, the reverse occurs. Blesa et al., (1994) calculated log Kso values for Fe oxides over the temperature range 25-300 °C from the free energies of formation for hematite, log iCso fell from 0.44 at 25 °C to -10.62 at300°C. 

The factors which influence the rate of dissolution of iron oxides are the properties of the overall system (e. g. temperature, UV light), the composition of the solution phase (e.g. pH, redox potential, concentration of acids, reductants and complexing agents) and the properties of the oxide (e. g. specific surface area, stoichiometry, crystal chemistry, crystal habit and presence of defects or guest ions). Models which take all of these factors into account are not available. In general, only the specific surface area, the composition of the solution and in some cases the tendency of ions in solution to form surface complexes are considered. 

New results of styrene formation over iron oxide single-crystal model catalysts were reported.326 In ultra-high-vacuum experiments with Fe304(lll) and a-Fe203(0001) films combined with batch reaction studies only Fe203 showed catalytic activity. The activity increased with increasing surface defect... 

Within inorganic photorefractive crystals the optical generation of carriers is associated usually with the oxidation of defect and impurity states within the optical band gap of the crystal, such as iron impurity within lithium niobate crystals, as in Figure 9. 

The iron oxides in natural surface environments are often poorly crystalline. i. e. the crystals are nano-sized (>100 nm), do not clearly exhibit the typical morphology of well-crystalline forms, are rich in defects and contain impurities. All this is most probably the result of their formation at low-temperature and in contaminated environments. Due to their striking colors (ranging from red to yellow) and their high surface area, small... 

Pores and macroscopic inclusions are three-dimensional crystal defects. From the standpoint of the reactivity of solids, pores can be very important. Consider, for instance, the formation of porous scales during oxidation (tarnishing) [11]. (For example, the decarburization of iron cannot occur if a non-porous oxide scale without grain boundaries is formed on its surface.) Or consider the direct reduction of ore [10] in which the reduction rate is greatly dependent upon the formation of porous metal surface layers. In many so-called solid state reactions, gaseous products are formed as well as solid reaction products as, for example, during the reaction of TiOa with BaCOs to produce BaTiOs with the formation of In such cases, just as in the case of ore reduction, the formation of a porous product surface layer is of decided importance for the progress of the reaction. 

The broadening of the spectrum upon treatment at 790 K is similar to observations made when calcium carbonate single crystals are subject to argon ion bombardment. The defects induced in the calcium oxide component of the catalyst cannot be caused by an initial reduction of the oxide, because calcium oxide is stable to hydrogen at 790 K. The defects may be indicative of the formation of a ternary calcium iron oxide since there is no observed increase in the dispersion of calcium, which would be associated with a disintegration of the CaO crystals. It should be pointed out that the other structural promoter element, aluminum, exhibits the same spectral changes in its A12 emission. The dispersion of the alumina increases, however, with reduction of the catalyst, particularly when the wet reduction method is applied (see Table 2.1). 

Nonstoichiometry may occur for some ceramic materials in which two valence (or ionic) states exist for one of the ion types. Iron oxide (wustite, FeO) is one such material because the iron can be present in both Fe " and Fe states the number of each of these ion types depends on temperature and the ambient oxygen pressure. The formation of an Fe " ion disrupts the electroneutrality of the crystal by introducing an excess -1-1 charge, which must be offset by some type of defect. This may be accomplished by the formation of one Fe " vacancy (or the removal of two positive charges) for every two Fe " ions that are formed (Figure 12.20). The crystal is no longer stoichiometric because there is one more O ion than Fe ion however, the crystal remains electrically neutral. This phenomenon is fairly common in iron oxide, and, in fact, its chemical formula is often written as Fei 0 (where x is some small and variable fraction substantially less than unity) to indicate a condition of nonstoichiometry with a deficiency of Fe. 

The discussion of the defects in FeO has so far been only structural. Now we turn our attention to the balancing of the charges within the crystal. In principle the compensation for the iron deficiency can be made either by oxidation of some Fe(II) ions or by reduction of some oxide anions. It is energetically more favourable to oxidise Fe(II). For each Fe vacancy, two Fe cations must be oxidised to Fe ". In the overwhelming majority of cases, defect creation involves changes in the cation oxidation state. In the case of metal excess in simple compounds, we would usually expect to find that neighbouring cation(s) would be reduced. 

The majority of inorganic systems reported to exhibit photochromism are solids, examples being alkali and alkaline earth halides and oxides, titanates, mercuric chloride and silver halides.184 185 The coloration is generally believed to result from the trapping of electrons or holes by crystal lattice defects. Alternatively, if the sample crystal is doped with an impurity capable of existing in variable oxidation states (i.e. iron or molybdenum), an electron transfer mechanism is possible. 

Simultaneous polymerization emd crystallization is another approach to memroscopic, defect-free single crystals of macromolecules (59). Recent examples include a preparative method for mixed metal coordination polymers (60), emd M. Hemack emd coworkers have reacted hemipotphyreizine (6 with iron (II) acetate in nitrobenzene to obtain single crystals of em oxygen-bridg polymer with iron in a -i- 4 oxidation state. 

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