Allotropism of iron

Figure 3.9. Simplified scfiematic of tfie transformation from BCC to FCC, exfiibited between the three allotropes of iron. Comer atoms have been omitted for clarity.

Some elements show polymorphism they can exist in more than one type of crystalline form. Iron is an example. At room temperature, iron exists as ferrite, or a-Fe, and is a magnetic body-centered cube. Between 906-1403 °C, however, an allotrope of iron (called y-Fe) exists in a nonmagnetic fece-centered cube. 

Crystalline Invar is face centred cubic (FCC) with a lattice constant of 3.59 A [18]. This corresponds to a nearest neighbour distance of -2.54 A. For comparison y-iron, the FCC allotrope of iron, which exists between 1,667 K and 1,185 K at atmospheric pressure, has a lattice constant [7] of 3.58 A. 

The name given to the body-centered cubic allotropes of iron, and to body-centered cubic solid solutions... 

Gobalt is a brittle, hard metal, resembling iron and nickel in appearance. It has a metallic permeability of about two thirds that of iron. Gobalt tends to exist as a mixture of two allotropes over a wide temperature range. The transformation is sluggish and accounts in part for the wide variation in reported data on physical properties of cobalt. 

Pure iron is a fairly soft silver/white ductile and malleable moderately dense (7.87 gcm ) metal melting at 1,535 °C. It exists in three allotropic forms body-centered cubic (alpha), face-centered cubic (gamma), and a high temperature body-centered cubic (delta). The average value for the lattice constant at 20 °C is 2.86638(19)A. The physical properties of iron markedly depend on the presence of low levels of carbon or silicon. The magnetic properties are sensitive to the presence of low levels of these elements, and at room temperature pure iron is ferromagnetic, but above the Curie point (768 °C), it is paramagnetic. 

Muller, J. Joubert, J.C. (1974) Synthese en milieu hydrothermal et caracterisation de Voxyhydroxyde de vanadium V OOH et d une nouvelle variete allotropic du dioxide VO2. J. Solid State Chem. 11 79—87 Muller, J.P. Bocquier, G. (1986) Dissolution of kaolinites and accumulation of iron oxides in lateritic-ferruginous nodules Mineralogical and microstructural transformations. Geoderma 37 113-136... 

The allotropy of elemental iron plays an important role in the formation of iron alloys. Upon solidification from the melt, iron undergoes two allotropic transformations (see Figure 2.9). At 1539°C, iron assumes a BCC structure, called delta-iron (5-Fe). Upon further cooling, this structure transforms to the FCC structure at 1400°C, resulting in gamma-iron (y-Fe). The FCC structure is stable down to 910°C, where it transforms back into a low-temperature BCC structure, alpha-iron (a-Fe). Thus, 5-Fe and a-Fe are actually the same form of iron, but are treated as distinct forms due to their two different temperature ranges of stability. 

Carbon is soluble to varying degrees in each of these allotropic forms of iron. The solid solutions of carbon in a-Fe, y-Fe, and <5-Fe are called, respectively, ferrite, austenite, and 8-ferrite. So, for example, the single-phase region labeled as y in... 

Three allotropic forms of iron are known (I) alpha irott. which is present below 769 C t2) grimnitr iron, which exists between 906 and 1.4(14 0, uid (3) delta imn, which occurs between 1,404 and l,536"C, On slow cooling, die reverse changes occur, but may be slowed or partly or entirely prevented in the presence ol alloying elemenls. 

Steel, as is well known, differs from iron by the presence of a certain amount of carbon, which induces the iron, when cold, to persist in its allotropic state. This appears to be due to a carbide of iron mixed with the excess of iron in the steel. The compound has been found as a meteoric mass it has been named cohenite, and has the formula FegC. On treating steel with dilute acetic acid, the same substance remains as a black powder. Its formula is similar to that of manganese carbide, MngC. 

Kaufman was with the Lincoln Laboratory at M. I. T. from 1955 to 1958, ManLabs from 1958 to 1988, and Alcan Aluminum Corporation from 1988 to 1996. Kaufman derived expressions for thermochemical lattice stabilities for allotropic and nonallotropic elements. He provided an early model for the thermodynamic description of iron, including magnetic contributions, and he was the founding editor of the CALPHAD journal. Kaufman is now a consultant and lecturer in the materials science and engineering department at M. I. T. 

Table 3.2. Uiut Cell Dimensions of Iron Allotropes and Fe—C Alloys ...

The particular advantage of diffraction analysis is that it discloses the presence of a substance as that substance actually exists in the sample, and not in terms of its constituent chemical elements. For example, if a sample contains the compound A By, the diffraction method will disclose the presence of A B as such, whereas ordinary chemical analysis would show only the presence of elements A and B. Furthermore, if the sample contained both A B, and Aj Bjy, both of these compounds would be disclosed by the diffraction method, but chemical analysis would again indicate only the presence of A and B. To consider another example, chemical analysis of a plain carbon steel reveals only the amounts of iron, carbon, manganese, etc., which the steel contains, but gives no information regarding the phases present. Is the steel in question wholly martensitic, does it contain both martensite and austenite, or is it composed only of ferrite and cementite Questions such as these can be answered by the diffraction method. Another rather obvious application of diffraction analysis is in distinguishing between different allotropic modifications of the same substance solid silica, for example, exists in one amorphous and six crystalline modifications, and the diffraction patterns of these seven forms are all different. 

Fig. 15. Crystalline structure of allotropic forms of iron. Each white sphere represents an atom of (a) a- and 5-iron in bcc form and (b) y-iron in fee form...

The allotropic form of oxygen, ozone, can also be employed for the oxidation of saturated hydrocarbons. The reactivity of ozone without additional reagents is not sufficient for the preparative functionalization of alkanes in solution however, its reactivity is increased substantially by the addition of iron(III) chloride [6] or antimony pentafluo-ride. [7] The dry ozonation variant [8] of Mazur et al. [9] by which alkanes are hydroxy-lated at tertiary C atoms with high selectivities and yields, was shown to be especially useful. According to this method, silica gel is coated with roughly 1 wt% of the substrate, cooled to -78 °C, saturated with ozone, and subsequently allowed to warm to room temperature within 0.5-2 h. Adamantane (1) is converted almost quantitatively into 1-adamantanol (4) in this way (Table 1), and this method of oxy-functionalization has been applied successfully even on certain steroids. [10]... 

PHYSICAL PROPERTIES steel-gray, shiny, hard metal ductile somewhat malleable hydrated salts of cobalt are red soluble salts form red solutions which become blue on adding concentrated hydrochloric acid exists in two allotropic forms hexagonal form is more stable than the cubic form at room temperature readily soluble in dilute nitric acid insoluble in water magnetic ferromagnetic permeability two-thirds that of iron MP (1493°C, 2719°F) BP (3100°C, 5612°F) DN (8.92 g/cm at 20°C) SG (8.92) CP (0.1056 cal/g/°C at 15-100°C) LHV (1500cal/g) VP (0 mmHg at 68°F BHN (1.25). 

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