Covalent Graphite Compounds

The structure of graphite oxide is still not clearly defined and several models have been proposed with various hydroxyl-, carbonyl-, ether-bridges and C=C bonds. The interlayer spacing is 0.6 - 0.7 nm. A typical structure is shown in Fig. 10.1 Graphite oxide is strongly hygroscopic and 

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Like the covalent graphite compounds, the intercalation compounds are formed by the insertion of a foreign material into the host lattice. The structure however is different as the bond, instead of being covalent, is a charge-transfer interaction. This electronic interaction results in a considerable increase in electrical conductivity in the ab directions. 

Covalent C—F bonds are also detectable in lamellar n charge-transfer graphite compounds such as CjSbFj C KrFj and halogen fluorides (see 16.4.2.1.1 and reviews ). These mixed covalent-rr charge-transfer compounds arise from internal fluorination by the intercalant. 

Overoxidation and solvolysis of graphite salts may yield covalent layered compounds with a fraction of sp carbon, e.g., graphite oxide (cf. 16.4.2.5), which is formed in HjO-containing acids . 

The metal is produced on a massive scale by the Hall-Heroult method in which aluminum oxide, a nonelectrolyte, is dissolved in molten cryolite and electrolyzed in a large cell. The bauxite contains iron oxide and other impurities, which would contaminate the product, so the bauxite is dissolved in hot alkali, the impurities are removed by filtration, and the pure aluminum oxide then precipitated by acidification. In the cell, molten aluminum is tapped off from the base and carbon dioxide evolved at the graphite anodes, which are consumed in the process. The aluminum atom is much bigger than boron (the first member of group 13) and its ionization potential is not particularly high. Consequently aluminum forms positive AT ions. However, aluminum also has nonmetallic chemical properties. Thus, it is amphoteric and also forms a number of covalently bonded compounds. 

Soiid Lubricants. A major application of covalent- and intercalated-graphite compounds is found in solid lubrication. The purpose of solid lubrication is to reduce friction and wear between surfaces in relative motion. The differences between graphite and liquid lubricants are shown in Table 10.6. 

Boron nitride is chemically unreactive, and can be melted at 3000 K by heating under pressure. It is a covalent compound, but the lack of volatility is due to the formation of giant molecules as in graphite or diamond (p. 163). The bond B—N is isoelectronic with C—C. 

The covalent compounds of graphite differ markedly from the crystal compounds. They are white or lightly colored electrical insulators, have Hi-defined formulas and occur in but one form, unlike the series typical of the crystal compounds. In the covalent compounds, the carbon network is deformed and the carbon atoms rearrange tetrahedraHy as in diamond. Often they are formed with explosive violence. 

Covalent compounds, arising from the attack of strong oxidizing systems, such as fluorine or Mn(VII), on graphite. The aromatic planarity of the graphite sheet is destroyed, and a buckled, sp -hybridized sheet is created. 

Although the covalent compounds of graphite are thus important in their own right, they represent the extreme form of oxidative intercalation. The use of fluoride compounds to achieve highly conductive materials may ultimately lead to new forms of graphite fluoride SI). 

If electron-pair, or covalent, bonding is periodic in two or three dimensions, crystals result. The most important case is the carbon-carbon bond. If it is extended periodically in two-dimensions the result is graphite in three-dimensions it is diamond. Other elements that form electron-pair bonds are Si, Ge, and a-Sn. Some binary compounds are A1P (isoelectronic with Si),... 

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