Iron, crystal structure

Blast furnace production of iron allows the hot, newly reduced product to trickle through the bed of heated coke to the hearth. Since carbon is somewhat soluble in molten iron, pig iron usually contains from 3 to 4.5% carbon. It also contains smaller percentages of other reduced elements such as silicon, phosphorus, manganese, etc., generated by the same reducing processes that yielded the iron (Table 14.3). Primarily from the effect of the high-carbon content on the iron crystal structure, the blast furnace product is brittle, hard, and possesses relatively low-tensile strength. Hence the crude pig iron product of the blast furnace is not much used in this form. 

An interesting phenomenon has been observed on reduction of iron. The surfactant chemistry has influenced the iron crystal structure. If the anionic surfactants (such as AOT) are used, we obtain a-Fe with a body-centered (bcc) crystal structure [198]. If the nonylphenol polyethoxylate surfactant is used, the crystals with the face-centered (fee) lattice are formed [199]. Similar data are known for metallic alloys. For this purpose, a mixture of metal salts has been subjected to the joint reduction [200]. It is essential that the reduction happens simultaneously with the formation of multication phases. 

The first crystal structure to be detennined that had an adjustable position parameter was that of pyrite, FeS2 In this structure the iron atoms are at the comers and the face centres, but the sulphur atoms are further away than in zincblende along a different tln-eefold synnnetry axis for each of the four iron atoms, which makes the unit cell primitive. 

Some metals have more than one crystal structure. The most important examples are iron and titanium. As Fig. 2.1 shows, iron changes from b.c.c. to f.c.c. at 914°C but goes... 

In order to answer these questions as directly as possible we begin by looking at diffusive and displacive transformations in pure iron (once we understand how pure iron transforms we will have no problem in generalising to iron-carbon alloys). Now, as we saw in Chapter 2, iron has different crystal structures at different temperatures. Below 914°C the stable structure is b.c.c., but above 914°C it is f.c.c. If f.c.c. iron is cooled below 914°C the structure becomes thermodynamically unstable, and it tries to change back to b.c.c. This f.c.c. b.c.c. transformation usually takes place by a diffusive mechanism. But in exceptional conditions it can occur by a displacive mechanism instead. To understand how iron can transform displacively we must first look at the details of how it transforms by diffusion. 

At 1 atmosphere, iron meits at 1536°C and boiis at 2860°C. When it sohdifies (a phase change), it does so in the b.c.c. crystai structure and is caiied d-iron. On cooiing further it undergoes two further phase changes. The first is at 1391 °C when it changes to the f.c.c. crystal structure, and is then called y-iron. The second is at 914°C when it changes baek to the b.c.c. crystal structure, and is called a-iron. 

PD Swartz, BW Beck, T Ichiye. Stiaictural origins of redox potential m iron-sulfur proteins Electrostatic potentials of crystal structures. Biophys 1 71 2958-2969, 1996. 

Finally, at even lower transformation temperatures, a completely new reaction occurs. Austenite transforms to a new metastable phase called martensite, which is a supersaturated solid solution of carbon in iron and which has a body-centred tetragonal crystal structure. Furthermore, the mechanism of the transformation of austenite to martensite is fundamentally different from that of the formation of pearlite or bainite in particular martensitic transformations do not involve diffusion and are accordingly said to be diffusionless. Martensite is formed from austenite by the slight rearrangement of iron atoms required to transform the f.c.c. crystal structure into the body-centred tetragonal structure the distances involved are considerably less than the interatomic distances. A further characteristic of the martensitic transformation is that it is predominantly athermal, as opposed to the isothermal transformation of austenite to pearlite or bainite. In other words, at a temperature midway between (the temperature at which martensite starts to form) and m, (the temperature at which martensite... 

Allotropy in the solid state can also arise because of differences in crystal structure. For example, solid iron has a body-centered cubic structure (recall Figure 9.16, page 246) at room temperature. This changes to a face-centered structure upon heating to 910°C. 

The validity of this approach can be demonstrated by the example of several complex fluoride compounds that exhibit ferroelectric properties, such as compounds that belong to the SrAlF5 family [402, 403]. The crystal structure of the compounds is made up of chains of fluoroaluminate octahedrons that are separated by another type of chains - ramified chains. Other examples are the compounds Sr3Fe2Fi2 and PbsWjOgFio. In this case, the chains of iron- or tungsten-containing octahedrons are separated from one another by isolated complexes with an octahedral configuration [423,424]. 

Analysis of the volumetric effects indicates that as a result of such mechanical activation, iron and manganese are concentrated in the extended part of the crystal, while tantalum and niobium are predominantly collected in the compressed part of the distorted crystal structure. It is interesting to note that this effect is more pronounced in the case of tantalite than it is for columbite, due to the higher rigidity of the former. Akimov and Chernyak [452] concluded that the effect of redistribution of the ions might cause the selective predominant dissolution of iron and manganese during the interaction with sulfuric acid and other acids. 

This dement is important mainly because of its use as an additive to iron in the manufacture of steel. A few percent of vanadium stabilizes a high-temperature crystal structure of iron so that it persists at room temperature. This form is tougher, stronger, and more resistant to corrosion than ordinary iron. Automobile springs, for example, are often made of vanadium steel. 

Analcite (NajOAljOj SiO I O), a cubic crystal structure, is formed at high temperatures. It is similar to acmite and also invariably is found beneath sludges of hydroxyapatite or serpentine or under porous deposits of iron oxides. 

Iron crystallizes in a bcc structure. The atomic radius of iron is 124 pm. Determine (a) the number of atoms per unit cell (b) the coordination number of the lattice (c) the length of the side of the unit cell. 

Reaction of iron atoms with cycloheptatriene to form [Fe( r) -C7H7)-(t7 -C7H9)] was confirmed by another group 15) these workers determined the crystal structure of the species, demonstrating a sandwich structure with the open faces of the two 7j -systems skewed to each other. The temperature-dependent NMR spectrum of this species (16) indicated two types of fiuxional behavior in solution. Evidence for a 1,-2-shift mechanism of the l-5-i7-cycloheptatrienyl moiety in the structure shown. 

X-ray absorption spectroscopy has been performed on the isolated Rieske protein from bovine heart mitochondrial bc complex 69) as well as on the Rieske-type cluster in Burkholderia cepacia phthalate dioxygenase (PDO) (72). The analysis performed by Powers et al. 69) was significantly hampered by the fact that the presence of two histidine ligands was not fully recognized therefore, only the results obtained with the dioxygenase where the mononuclear iron has been depleted will be considered here. Table VII gives a comparison of the distances obtained from the fit of the EXAFS spectra assuming an idealized Rieske model and of the distances in the crystal structures... 

While crystal structures of rubredoxins have been known since 1970 (for a full review on rubredoxins in the crystalline state, see Ref. (15)), only recently have both crystal and solution structures of Dx been reported (16, 17) (Fig. 3). The protein can be described as a 2-fold symmetric dimer, firmly hydrogen-bonded and folded as an incomplete /3-barrel with the two iron centers placed on opposite poles of the molecule, 16 A apart. Superimposition of Dx and Rd structures reveal that while some structural features are shared between these two proteins, significant differences in the metal environment and water structure exist. They can account for the spectroscopic differences described earlier. 

The 3D crystal structure of Dsm. baculatum [NiSeFe] hydrogenase has been solved 185), and it was indicated that the enzyme contains three [4Fe-4S] centers. A cysteine (replacing a proline usually found near the [3Fe-4S] core) provides an extra ligand, enabling the acceptance of a fourth iron site at this cluster. 

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