Graphite allotropism

Similar educational opportunities abound for carbon. The diamond and graphite allotropes of carbon have been mainstays of chemistry classes for generations of students and provide a contrast between a three-dimensional structure of great hardness and a two-dimensional structure with lubricant properties, respectively. We now have what can be regarded as zero- and onedimensional counterparts - buckyballs and carbon nanotubes, respectively - with their rich diversity of structural relatives and physicochemical properties (4). These materials are being employed in a variety of nanoscale devices because of their unusual chemical, mechanical and electrical properties. 

Black is sometimes the appearance of carbon in its graphite allotropic form. 

In Chapter 6, we discussed the structure of diamond and graphite—allotropes of carbon. The discovery of the Ceo molecule in 1985 introduced a new, third, ordered form of carbon—the fullerenes. The special new feature of Ceo (shown in Figure 7.16a) is the regular incorporation of five-sided rings of C atoms that allows the formation of curved sheets of carbon atoms. Ceo has 12 pentagonal (five-sided) and 20 hexagonal (six-sided) faces... 

The crystal structure of silicon is similar to that of diamond however, the Si—Si bonds (226 kJ/mol) are weaker than the C—C bonds (356 kJ/mol), and silicon is not nearly as hard. There is no graphitic allotrope of silicon. Crystalline silicon is a blue-gray, somewhat shiny, brittle element that certainly appears metallic however, it is classified as a nonmetal or metalloid because it is a semiconductor that is, at low temperatures it is an insulator. However, when heated sufficiently, its electrical conductivity increases markedly. Very pure silicon for transistors is produced by reducing silicon tetrachloride prepared by... 

FIGURE 6.9 A phase diagram of carbon, showing where the graphite allotrope is stable and where the diamond allotrope is stable. 

Carbon is found free in nature in three allotropic forms amorphous, graphite, and diamond. A fourth form, known as "white" carbon, is now thought to exist. Ceraphite is one of the softest known materials while diamond is one of the hardest. 

The eommonest erystalline forms of earbon, cubie diamond and hexagonal graphite, are elassical examples of allotropy that are found in every chemistry textbook. Both diamond and graphite also exist in two minor crystallographie forms hexagonal diamond and rhombohedral graphite. To these must be added earbynes and Fullerenes, both of which are crystalline earbon forms. Fullerenes are sometimes referred to as the third allotrope of carbon. However, sinee Fullerenes were diseovered more recently than earbynes, they are... 

Despite many publications on carbynes, their existence has not been universally accepted and the literature has been characterised by conflicting claims and counter claims [e.g., 27-29]. This is particularly tme of meteoritic carbynes. An interesting account of die nature of elemental carbon in interstellar dust (including diamond, graphite and carbynes) was given by Pillinger [30]. Reitmeijer [31] has re-interpreted carbyne diffraction data and has concluded that carbynes could be stratified or mixed layer carbons with variable heteroelement content (H,0,N) rather than a pure carbon allotrope. 

Chapter 1 contains a review of carbon materials, and emphasizes the stmeture and chemical bonding in the various forms of carbon, including the foui" allotropes diamond, graphite, carbynes, and the fullerenes. In addition, amorphous carbon and diamond fihns, carbon nanoparticles, and engineered carbons are discussed. The most recently discovered allotrope of carbon, i.e., the fullerenes, along with carbon nanotubes, are more fully discussed in Chapter 2, where their structure-property relations are reviewed in the context of advanced technologies for carbon based materials. The synthesis, structure, and properties of the fullerenes and... 

Fig. 4. EEL spectra of (a) graphite and (b) diamond. These carbon allotropes represent different spectra sp bonding especially exhibits 7c -excitation peak lower than the o -excitation peaks (modified from ref. 16).

Quite apart from the fullerene cluster molecules, numerous other molecular allotropes of carbon, C , have been discovered in the gases formed by the laser vaporization/supersonic expansion of graphite. The products are detected by mass... 

Pencil lead" is almost pure graphite. Graphite is the stable elemental form of carbon at 25°C and 1 atm. Diamond is an allotrope of graphite. Given... 

Allotrope One of two or more forms of an element in the same physical state. Graphite and diamond are allotropes of carbon 02 and 03 are allotropes of oxygen, 250... 

What I hope to have added to the discussion has been a philosophical reflection on the nature of the concept of element and in particular an emphasis on elements in the sense of basic substances rather than just simple substances. The view of elements as basic substances, is one with a long history. The term is due to Fritz Paneth, the prominent twentieth century radio-chemist. This sense of the term element refers to the underlying reality that supports element-hood or is prior to the more familiar sense of an element as a simple substance. Elements as basic substances are said to have no properties as such although they act as the bearers of properties. I suppose one can think of it as a substratum for the elements. Moreover, as Paneth and before him Mendeleev among others stressed, it is elements as basic substances rather than as simple substances that are summarized by the periodic table of the elements. This notion can easily be appreciated when it is realized that carbon, for example, occurs in three main allotropes of diamond, graphite and buckminsterfullenes. But the element carbon, which takes its place in the periodic system, is none of these three simple substances but the more abstract concept of carbon as a basic substance. 

The substance indicated by the same symbol in two or more equations is in exactly the same state in the reactions represented by those equations. In particular, the different allotropic modifications of a solid element (e.g., charcoal, graphite, diamond or yellow and red phosphorus) have different heats of combustion, and the particular form used must be specified in every case. 

Self-Test 7.1 IB Use the information in Table 7.3 or Appendix 2A to determine which allotrope is the more ordered form and predict the sign of AS for each transition (a) white tin (Fig. 7.14) changes into gray tin at 25°C (b) diamond changes into graphite at 25°C. 

Solid carbon exists as graphite, diamond, and other phases such as the fullerenes, which have structures related to that of graphite. Graphite is the thermodynamically most stable of these allotropes under ordinary conditions. In this section, we see how the properties of the different allotropes of carbon are related to differences in bonding. 

We can understand the differences in properties between the carbon allotropes by comparing their structures. Graphite consists of planar sheets of sp2 hybridized carbon atoms in a hexagonal network (Fig. 14.29). Electrons are free to move from one carbon atom to another through a delocalized Tr-network formed by the overlap of unhybridized p-orbitals on each carbon atom. This network spreads across the entire plane. Because of the electron delocalization, graphite is a black, lustrous, electrically conducting solid indeed, graphite is used as an electrical conductor in industry and as electrodes in electrochemical cells and batteries. Its... 

Carbon has an important series of allotropes diamond, graphite, and the fullerenes. 

Compare the hybridization and structure of carbon in diamond and graphite. How do these features explain the physical properties of the two allotropes ... 

This chapter is a review of the two major allotropes graphite and diamond, which are both produced extensively by CVD. The properties of these two materials can vary widely. For instance, diamond is by far the hardest-known material, while graphite can be one of the softest. Diamond is transparent to the visible spectrum, while graphite is opaque diamond is an electrical insulator, while graphite is a conductor. 

There are more than a million known carbon compounds, of which thousands are vital to life processes. The carbon atom s unique and characteristic ability to form long stable chains makes carbon-based life possible. Elemental carbon is found free in nature in three allotropic forms amorphous carbon, graphite, and diamond. Graphite is a very soft material, whereas diamond is well known for its hardness. Curiosities in nature, the amounts of elemental carbon on Earth are insignificant in a treatment of the... 

The carbon-based nanofillers are mainly layered graphite, nanotube, and nanofibers. Graphite is an allotrope of carbon, the stmcture of which consists of graphene layers stacked along the c-axis in a staggered array [1], Figure 4.1 shows the layered structure of graphite flakes. 

The state of research on the two classes of acetylenic compounds described in this article, the cyclo[ ]carbons and tetraethynylethene derivatives, differs drastically. The synthesis of bulk quantities of a cyclocarbon remains a fascinating challenge in view of the expected instability of these compounds. These compounds would represent a fourth allotropic form of carbon, in addition to diamond, graphite, and the fullerenes. The full spectral characterization of macroscopic quantities of cyclo-C should provide a unique experimental calibration for the power of theoretical predictions dealing with the electronic and structural properties of conjugated n-chromophores of substantial size and number of heavy atoms. We believe that access to bulk cyclocarbon quantities will eventually be accomplished by controlled thermal or photochemical cycloreversion reactions of structurally defined, stable precursor molecules similar to those described in this review. 

Ceo and higher fullerenes are distinguished from other allotropes of carbon, diamond and graphite, in that they exist as discrete molecules. The spherical or ellipsoidal nature of the monotropes opens up the possibility of intriguing new areas of chemistry. Here we are only interested in the hydrogen (or muonium) adducts, although this study has important implications to the very vigorous and extensive research in fullerene chemistry. 

Potassium violently reacts with carbon in most of its allotropic forms. Thus the action of molten metal on graphite results in the formation of some kind of a carbide ... 

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