Diamond Crystal Forms

Diamond occurs in several crystal forms (or habits) which include the octahedron, the dodecahedron, and others which are more complicated. As a reminder, the simple crystallographic planes (100,110 and 111) in a cubic crystal are shown in Fig. 11.4.[ l 

These simple planes correspond to the faces of the three major crystal forms of diamond the 100 cubic, the 110 dodecahedral and the 111 octahedral (Fig. 11.5). Both cubic and octahedral surfaces predominate in high-pressure synthetic diamond where they are found alone or in combination to form blocky crystals.  

In CVD diamond, the (111) octahedral and the (100) cubic surfaces predominate and cubo-octahedral crystals combining both of these surfaces are commonly found. Twinning occurs frequently on the (111) surface. Faceted crystals of cut diamonds are predominantly the (111) and (100) surfaces. 

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Crystallization conditions such as temperature, solvent, and concentration can influence crystal form. One such modification is the truncation of the points at either end of the long diagonal of the diamond-shaped crystals seen in Fig. 4.11b. Twinning and dendritic growth are other examples of such changes of habit. 

In a typical use of this method, a mixture of hydrogen and methane is fed into a reaction chamber at a pressure of about 1.33 kPa (10 torr). The substrate upon which diamond forms is at about 950°C and Hes about 1 cm away from a tungsten wine at 2200°C. Small diamond crystals, 1 mm or so in si2e, nucleate and grow profusely on the substrate at a rate around 0.01 mm /h to form a dark, rough polycrystalline layer with exposed octahedral or cubic faces, depending on the substrate temperature. 

There are four allotropic forms of manganese, which means each of its allotropes has a different crystal form and molecular structure. Therefore, each allotrope exhibits different chemical and physical properties (see the forms of carbon—diamond, carbon black, and graphite). The alpha (a) allotrope is stable at room temperature whereas the gamma (y) form is soft, bendable, and easy to cut. The delta A allotrope exists only at temperatures above 1,100°C. As a pure metal, it cannot be worked into different shapes because it is too brittle. Manganese is responsible for the color in amethyst crystals and is used to make amethyst-colored glass. 

Diamonds are another allotrope whose crystal structure is similar to graphite. Natural diamonds were formed under higher pressure and extreme temperatures. Synthetic diamonds have been artificially produced since 1955. 

Diamond is crystallized in cubic form (O ) with tetrahedral coordination of C-C bonds around each carbon atom. The mononuclear nature of the diamond crystal lattice combined with its high symmetry determines the simplicity of the vibrational spectrum. Diamond does not have IR active vibrations, while its Raman spectrum is characterized by one fundamental vibration at 1,332 cm . It was found that in kimberlite diamonds of gem quality this Raman band is very strong and narrow, hi defect varieties the spectral position does not change, but the band is slightly broader (Reshetnyak and Ezerskii 1990). 

The formation of a Si crystal is shown in Fig. 1.10. Aside from the core, each Si atom has four valence electrons two 3s electrons and two 3p electrons. To form a Si crystal, one of the 3s electrons is excited to the 3p orbital. The four valence electrons form four sp hybrid orbitals, each points to a vertex of a tetrahedron, as shown in Fig. 1.10. Thpse four sp orbitals are unpaired, that is, each orbital is occupied by one electron. Since the electron has spin, each orbital can be occupied by two electrons with opposite spins. To satisfy this, each of the directional sp orbitals is bonded with an sp orbital of a neighboring Si atom to form electron pairs, or a valence bond. Such a valence bonding of all Si atoms in a crystal form a structure shown in (b) of Fig. 1.10, the so-called diamond structure. As seen, it is a cubic crystal. Because all those tetrahedral orbitals are fully occupied, there is no free electron. Thus, similar to diamond, silicon is not a metal. 

Varying the conditions of deposition of the film in CVD can alter the morphology of the nanocrystals formed Figure 11.1(a) and Figure 11.1(b) show nanosized diamond crystals in diamond films grown with 111 (octahedral) and 100 (cubic) faces. Techniques for producing specific morphologies could be very important in the production of catalysts because different crystal faces can catalyse very specific reactions. 

The rounded forms of natural diamond crystals are commonly observed in crystals occurring both in alluvial deposits (secondary deposits) and in mother rocks... 

There is a great deal of evidence that demonstrates that natural diamond crystals were partially dissolved, and this is summarized in Table 9.2. Special attention should be paid to the superimposed circular ditches (Fig. 9.6). This patterning may be explained by assuming that bubbles, formed by degassing during the uplifting process of the magma, adhered on the crystal surface and resisted dissolution. 

In the morphology of as-grown micro-diamond crystals, no essential difference was detected between T5tpe I and II crystals, which both take octahedral forms. This is clearly shown in Fig. 9.21 [21], in which the two t)tpes are compared by the transmittance of ultra-violet rays. There is no essential difference in morphology between Type II, which is transparent under the ultra-violet ray used, and Type I, which is opaque to the same wavelength. 

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