Allotropic modifications of carbon

After different allotropic modifications of carbon nanostructures (fullerenes, tubules) have been discovered, a lot of papers dedicated to the investigations of such materials, for instance [9-15] were published, determined by the perspectives of their vast application in different fields of material science. 

Since the discovery of fullerenes the scientific community has been taking an active interest in peculiarities of the fullerene formation and structure, physical and chemical properties. The fourth allotropic modification of carbon (fullerene) is unique molecule having a spatial structure with icosahedral symmetry and showing distinctive properties in interaction with other substances. Under certain conditions fullerenes can accept and donate hydrogen atoms to form hydrofullerenes. 

Tucker and Moody9 were unable to prepare the almost pure carbide described by Moissan, and attributed their failure to the very small temperature-interval between the formation and the decomposition of the substance. The carbide is also formed by the interaction of lithium and any of the allotropic modifications of carbon in vacuum at dull red heat and by the combination of the metal with carbon monoxide or dioxide, or with ethylene or acetylene, an impure product is obtained.10... 

With this background, it should be clear that nanostructure is what defiues, in classical terminology, the three allotropic modifications of carbon materials ... 

Soon after the synthesis of diamond Soviet scientists prepared a new substance, carbine, which, as has since been proved, is a new, third allotropic modification of carbon. The carbon atoms in it comprise long chains. This substance resembles soot. 

Structures of organic molecules can be drawn from the stfuctures in which carbon appears as an element i.e. from the structures of its allotfopic modifications. Until the last quarter of the twentieth century only two allotropic modifications of carbon were known, graphite and diamond. From the viewpoint of structural organic chemistry, stractures of graphite and diamond represent basic stfuctural patterns by which carbon atoms can be intercoimected. 

The scope of tire following article is to survey the physical and chemical properties of tire tliird modification of carbon, namely [60]fullerene and its higher analogues. The entluisiasm tliat was triggered by tliese spherical carbon allotropes resulted in an epidemic-like number of publications in tire early to mid-1990s. In more recent years tire field of fullerene chemistry is, however, dominated by tire organic functionalization of tire highly reactive fullerene... 

Allotropic forms of carbon. In the solid state, the element carbon exists in three different allotropic modifications—amorphous carbon and the two crystalline forms known as diamond and graphite. Amorphous carbon includes numerous common products such as wood charcoal, bone black, coke, lamp black, and carbon black. Each of these varieties of crystalline and amorphous carbon possesses properties that render it useful for a variety of purposes. 

Allotropic modifications of an element bear the name of the atom from which they are derived, together with a descriptor to specify the modification. Common descriptors are Greek letters (a, (3, y, etc.), colours and, where appropriate, mineral names (e.g. graphite and diamond for the well known forms of carbon). Such names should be regarded as provisional, to be used only until structures have been established, after which a rational system based on molecular formula (see Section IR-3.4.3) or crystal structure (see Section IR-3.4.4) is recommended. Common names will continue to be used for amorphous modifications of an element and for those which are mixtures of closely related structures in their commonly occurring forms (such as graphite) or have an ill-defined disordered structure (such as red phosphoms) (see Section IR-3.4.5). 

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. 

Contrasting the classical modifications of carbon discussed in Chapter 1, fuUer-enes are discrete molecules, which clearly refiects in their properties. Above all, the spectroscopic features show significant differences to those of other carbon allotropes. 

AixotrOiw,—Dimorq hism apart, a few substances are known to exist in more than one solid form. These varieties of the same substance exhibit different physical properties, while their chemical qualities are the same in kind Such m ifications are said to be affoiropic. One or more allotropic modifications of a substance are usually crtjifidliyiet the other or others amorphous or vitreous. Sulplmr, for example, exists not only in two dimorphous varieties of ciy stols, but also in a third, allotropic form, in which it is flexible, amorphous, and transparent. Carbon exists in three allotropic forms two crystaUine, the diamond and graphite the Ihirtl amorphous. 

Lea produced his allotropic silver (4) by treating in the cold 200 cc. of a 10 per cent silver nitrate solution with a mixture consisting of 200 cc. of 30 per cent ferrous sulfate, 250 cc. of a 40 per cent sodium citrate solution, and 50 cc. of a 10 per cent sodium carbonate solution. The violet precipitate was filtered and washed in water. In order to remove the impurities the precipitation was repeated a number of times with ammonium nitrate. Finally the solution was evaporated and a mass with a metallic luster obtained that consisted mostly of silver. Lea found that the metal did not diffuse through membranes and that it could be freed by this means from electrolytes. Because of these properties and also because inorganic colloids were considered to be allotropic modifications of the metals in question, Lea decided that his silver must also be an allotropic form. Even today we are not in a position to deny this assertion. Barus and Schneider f have shown that it is not at all necessary to assume allotropic modifications in order to explain the subdivision in water or the behavior of the colloid toward electrolytes. We have therefore no direct evidence that the metal in the colloidal state is not an allotropic modification but the assumption is quite unnecessary and perhaps improbable. On the other hand recent work has shown that allotropy is not so uncommon as it was previously supposed, and if the rule proposed by W. Oswald holds, that the more unstable form appears first, it may very well be that colloidal metals contain, or are, allotropic modifications. 

A number of chemical elements, mainly oxygen and carbon but also others, such as tin, phosphorus, and sulfur, occur naturally in more than one form. The various forms differ from one another in their physical properties and also, less frequently, in some of their chemical properties. The characteristic of some elements to exist in two or more modifications is known as allotropy, and the different modifications of each element are known as its allotropes. The phenomenon of allotropy is generally attributed to dissimilarities in the way the component atoms bond to each other in each allotrope either variation in the number of atoms bonded to form a molecule, as in the allotropes oxygen and ozone, or to differences in the crystal structure of solids such as graphite and diamond, the allotropes of carbon. 

If you stick to the definition of an allotrope being a modification of an element characterized by its x-ray crystal structure. Otherwise carbon may have more modifications, when counting all the different fullerenes and carbon nanotubes as allotropes. 

However, Briegleb,6 from X-ray investigations, maintains that in all the allotropic modifications two such pseudo-components do exist, and that these may be separated in some degree by taking advantage of the fact that although their absolute solubilities in carbon disulphide are almost identical, the rates at which they dissolve are different. By spectroscopic methods evidence has been obtained that the two forms exist in equilibrium in this solution and that the equilibrium varies with the temperature. 

Allotropic forms of phosphorus. Solid phosphorus exists in two distinct allotropic modifications and is also commonly encountered in a form consisting of a mixture of the two. White (or yellow) phosphorus is a translucent, waxlike solid which melts at 44°C, boils at about 290°C, and has a density of 1.83. When vaporized, the resulting gas consists of tetraatomic molecules (P4) up to a temperature of about 1500°C, whereupon these molecules partly dissociate into (and exist in equilibrium with) diatomic molecules (P2). White phosphorus is insoluble in water but is soluble in solvents such as ethyl ether and carbon disulfide. Great care should always be exercised in handling this form of phosphorus since it is highly flammable and very poisonous. Skin burns caused by phosphorus are exceedingly painful and very slow to heal. 

Allotropic forms of sulfur. Solid sulfur exists in two crystalline modifications. Rhombic sulfur consists of S8 molecules, is stable at temperatures below 95.5°C, has a specific gravity of 1.96, and is soluble in carbon disulfide. At 95.5°C, rhombic sulfur changes slowly, with absorption of heat, into the monoclinic form. Molten rhombic sulfur consists of S8 molecules and exists as a pale-yellow, thin, and limpid liquid known as X sulfur. When the temperature is raised, X sulfur is slowly converted to dark and viscous p sulfur, which consists of Ss and S4 molecules and which is considered to be the amorphous variety of... 

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