Bunched spirals

Fig. 7.2. Projections of (a) simple spiral, (b) bunched spiral and (c) a bunched spiral with a moment modulation [after Felcher et al. (1976)].

If spiral growth occurs due to the existence of screw dislocations, the results depend upon whether the diffusion length ijy is smaller or larger than the typical separation of the spiral arms i. In the first case the situation hardly changes from the purely kinetic situation without diffusion, but in the second case interaction between steps comes into effect [90] and phenomena such as step bunching [91] take place. We can estimate qualitatively the... 

It is often observed in tiny crystals of micrometer order, such as clay minerals, that the entire surface of a crystal face is covered by elementary spiral layers originating from one screw dislocation (Fig. 5.3). Figure 5.10(a) shows such an exceptional case observed on a (0001) face of a SiC crystal synthesized by the Acheson method. However, such a situation is almost exceptional on crystal faces larger than millimeter size, and is encountered only on crystals synthesized under very precisely controlled conditions. In general, there are many growth centers on one crystal face, and steps from these centers bunch together to form macro-steps, which constitute the step patterns of the face. 

When we observe the process of advancement of elemental spiral steps in situ, it is often noticed that two steps bunch together to form a step with the height of two layers as they advance. The advancing rate of the bunched layer is retarded... 

Figure 5.17. Phase contrast photomicrograph showing bunching of elemental spiral steps (arrows) in hematite, (0001) face.

Polycrystalline Bunched Layers, Ridges Spirals, Layers, Blocks (well formed)... 

Ho 10 20 133 Bunched cone structure Basal plane spiral... 

Ho hep Tn = 133K basal plane spiral bunched basal plane spiral Tc = 20 K bunched cone structure, semiangle —80° Is 4.50... 

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