9.4 Morphology of single crystals 183
Figure 9.12. Growth sector showing a center-cross pattern formed by the growth of
smooth {111} and rough {100} interfaces.
growth, and the crystal eventually takes a simple octahedral form. Whereas the growth banding in the {111} growth sectors is straight and parallel to {111}, that in the {100} growth sectors is wavy and curved, like a hammock. This indicates that on the same crystal {111} behaves as a smooth interface (F face), whereas {100} behaves as a rough interface (K face). In natural diamond crystals, {100} behaves exclusively as a rough interface, and no evidence has been obtained so far to show that {100} may sometimes behave as a smooth interface.
From these observations, we may conclude that, in the growth of natural diamond crystals, three faces, {111}, {110}, and {100}, behave and show characteristics completely in agreement with the characteristics expected from PBC analysis. Therefore, we may conclude that, under the environmental conditions of natural diamond growth (principally in the silicate solution phase), {111} always behaves as a smooth interface under /kT * conditions, whereas the /kT ** of {110}, and particularly of {100}, stays close to the origin under any conditions, and these faces behave exclusively as rough interfaces.
Figure 9.13 shows morphological changes of diamond crystals on a diagram of growth rate versus driving force relation (see Fig. 3.15), based on the relation of the ** positions of the {111} and {100} faces. From this, it is expected that spherulitic forms and cuboids occur under higher driving force conditions, whereas octahedral crystals are expected under lower driving force conditions.
9.4.4Different solvents (synthetic diamond)
In the case of synthetic diamond, grown under high-temperature, highpressure conditions from a high-temperature solution with metal or alloy as the solvent, diamond crystals exhibit a cubo-octahedral Tracht bounded by {100} and
184 Diamond
growth rate R
driving force
Figure 9.13. Morphology of natural diamond crystals expected in relation to the driving force. The morphological change is as expected assuming that the positions of
/kT * and /kT ** are different for {111} and {100}.
{111}. Although the development of these two faces may vary depending on the driving force, both show evidence of spiral growth, as can be seen in Fig. 9.14. This indicates that {100} transforms from a rough K face to a smooth F face in a metallic solution. A transformation from a K face to an F face requires a change in the surface structure of the {100} face (i.e. surface reconstruction should take place), resulting in the same effect as introducing a new PBC. In the growth of natural diamond in a high-temperature solution phase with silicate as the solvent, surface reconstruction does not occur, whereas it does occur if a metal solvent is used. Surface reconstruction occurs in a solution in which the metallic element has a small ionic radius as the solvent, whereas it does not occur in a solvent with large ionic radius.
Recently, diamond synthesis has been successfully performed under hightemperature, high-pressure conditions in a system using kimberlite powder, various carbonates, sulphates or water as the solvent [13], [14]. Higher pressure and temperature conditions are required in a non-metallic solution than in a metallic solution, and the crystals obtained are mainly simple octahedral, differing from those observed in crystals grown from metallic solutions. Crystals synthesized in a non-metallic solution show the same characteristics as natural diamond Tracht. These observations indicate that the solvent components have a definitive effect upon surface reconstruction, and thus on the morphology of the crystals.
9.4 Morphology of single crystals 185
(a)
(b)
Figure 9.14. Spiral growth layers observed on (a) {100} and (b) {111} faces of diamond
crystals synthesized under high-pressure and high-temperature conditions.
186 Diamond
(a)
(b)
Figure 9.15. Comparison of cathodoluminescence tomographs of (a) natural and
(b) synthetic diamonds. (By courtesy of All Japan Gemmological Association [15].)
The resulting differences in the morphology of natural and synthetic diamond single crystals using metal solvents is also recorded in patterns shown by growth sectors and growth banding. If we reveal inhomogeneities of this sort using such techniques as cathodoluminescence, the differences between natural and synthetic diamonds are easily distinguishable. Figure 9.15 is an example of cathodoluminescence photographs of natural and synthetic diamonds grown in metallic solution, and the difference between the two is very evident [15]. It is noted that the intensity of the cathodoluminescence is remarkably different depending on growth sector. Figure 9.15 also indicates that element partitioning is affected by crystallographic directions, and therefore by kinetics.
Vapor grown diamonds are synthesized by the CVD method under low-pressure conditions, for which diamond becomes unstable, and it is possible to obtain single crystals of micrometer size. Frequent occurrences of multiply twinned par-