Lonsdaleite carbon allotropes

Figure 15.1 Three major allotropes of carbon (1 to r) diamond, lonsdaleite, and graphite.

Carbynes are white solids made up of carbon atoms with sp hybridization. Two main possibilities [23] exist for this Hnear structure polyine (—C=C—C=C—) and polycumulene (=C=C=C=C=) theoretical predictions and experimental evidence also point to the existence of cycHc carbynes [10]. According to a classification based on the type of bond present (Table 2.1), and also according to chronology, carbynes (rather than fuUerenes) should be considered as the third aUotropic form of carbon. Our scarce knowledge of carbynes and doubts about whether they really exist in pure form [10, 15, 26] are factors that have contributed to this erroneous interpretation. Moreover, lonsdaleite and rhombohedral graphite should be considered as polytypes rather than new structures with a different equation of state [27]. Strictly speaking, therefore, they should not be regarded as true allotropes of carbon (Table 2.1). 

Other recently discovered allotropes include a nanofoam—z web of light magnetic carbon clusters, lonsdaleite—a sort of disfigured hexagonal diamond lattice, and an aggregated diamond nanorod. 

Fig. 5.1 Eight allotropes of carbon a Diamond, b Graphite, c Lonsdaleite, d C60 (Buckminsterfullerene or buckyball), e C540, f C70, g Amorphous carbon, and h single-walled carbon nanotube or buckytube [25]...

These allotropic solids can be classified into three major categories (i) the sp structures which include graphite, the graphitic materials, amorphous carbon, and other carbon materials (all reviewed in Ch. 3), (ii) the sp structures which include diamond and lonsdaleite (a form detected in meteorites), reviewed in Ch. 11, and (iii) he Fullerenes (see Ch. 15). 

Eight allotropes of carbon (a) Diamond, (b) Graphite, (c) Lonsdaleite, (d) Ceo (Buckminster-fullerene or buckyball), (e) C54o, (f) C70, (g) Amorphous carbon, and (h) single-walled carbon nanotube or buckytube (The picture adopted from Wikipedia - http //en.wikipedia.org/wiki/ Allotropes of carbon)... 

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