Screw dislocation kinks

Figure 4.2 Terraces, ledges and kinks on a solid surface, together with an emerging screw dislocation, a vacant site, and an adatom...

By their nature, dislocations cannot end suddenly in the interior of a crystal a dislocation line can only end at a free surface or a grain boundary (or form a closed loop). Where a screw dislocation intersects a free surface there is inevitably a step or ledge in the surface, one atomic layer high, as shown in Fig. 20.30c. Furthermore, the step need not necessarily be straight and will, in fact, almost certainly contain kinks. 

Figure 4.2 Quasi-hexagonal dislocation loop lying on the (111) glide plane of the diamond crystal structure. The <110> Burgers vector is indicated. A segment, displaced by one atomic plane, with a pair of kinks, is shown a the right-hand screw orientation of the loop. As the kinks move apart along the screw dislocation, more of it moves to the right.

Figure 5.1 shows a schematic elevation through a kink on a screw dislocation in the diamond crystal structure. The black circles lie in the plane of the figure. The white ones lie in a plane in front of the figure, and the gray ones in a plane behind the figure. The straight lines represent electron pair bonds... 

Figure 5.9 Plan view of the (111) plane of the diamond structure. A—Normal structure with open circles in the plane of the paper, and crossed circles in the plane above. Each pair is connected by a covalent bond. B—Partial shear of the upper plane over the lower one on the right-hand side creating a screw dislocation line with a kink in it (dashed line). C—Upper plane sheared down-ward by the displacement, b.

Figure 5.10 Schematic dislocation core. Arrangement at kink on screw dislocation line.

Since the evaporation of a solid would occur at the kink sites because the bonding is weaker, atoms would diffuse also to these sites before evaporation. A demonstration of this is to be found on the morphologies of single crystals after a period of heating in vacuum to cause substantial evaporation. The resultant surface shows an increase in the number of ledges and kinks relative to the area of the terraces. It is also to be expected that dislocations emerging at the surface of catalysts, either as edge or screw dislocations, would play a... 

The classical crystal growth theory goes back to Burton, Cabrera and Frank (BCF) (1951). The BCF theory presents a physical picture of the interface (Fig. 6.9c) where at kinks on a surface step - at the outcrop of a screw dislocation-adsorbed crystal constituents are sequentially incorporated into the growing lattice. 

Figure 3.16. Some simple defects found on a low-index crystal face 1, the perfect flat face, a terrace 2, an emerging screw dislocation 3, the intersection of an edge dislocation with the terrace 4, an impurity adsorbed atom 5, a monatomic step in the surface, a ledge 6, a vacancy in the ledge 7, a kink, a step in the ledge 8 an adatom of the same type as the bulk atoms 9, a vacancy in the terrace 10, an adatom on the terrace. (From Ref. 12, with permission from Oxford University Press.)...

Steps, In a real crystal where dislocations are present, there are two types of steps the step that begins and ends on the boundary of the surface (Fig. 3.13a), and the step that starts on the surface and terminates on a boundary (Fig. 3.4). If a step starts on a surface, this is a place where a screw dislocation meets the surface. At 0 K, steps tend to be straight, but as the temperature is raised (F > 0 K), step roughness develops and the structure of the step includes a number of kinks, adsorbed atoms (adatoms or adions), and vacancies (Fig. 3.16). Steps can be of monatomic height or, as in the case of a real crystal surface, polyatomic height. 

One has, therefore, a picture of ions from a solution being transferred onto the electrode surface as adions of adions joining steps, kinks, etc. of steps advancing on the surface of screw dislocations yielding growth spirals of the surface advancing... 

The relevance of crystal faces to the subject of electrociystalhzation comes up as follows Each of the crystal faces just described contains all the microfeatures that have been described in previous sections, steps, kinks, etc. Further, the same phenomena of deposition—the ions crossing the electrified interface to form adions, the surface diffusion, lattice incorporation of adions, screw dislocation, growth spirals, etc.—occur on all the facets. 

Fig. 1. Surface structure often found on low-index crystal faces. 1, A terrace perfectly flat crystal face. 2, An emerging screw dislocation. 3, The intersection of an edge dislocation with a terrace. 4, A ledge or monatomic step, 5. A kink a step in a ledge. 6, A vacancy in a ledge. 7, An adsorbed growth unit on a ledge.

Figure 13.27. Surface models for crystal growth (a) mononuclear growth, (b) polynuclear growth, and (c) screw dislocation growth. Along the step a kink site is shown. Adsorbed ions diffuse along the surface and become preferentially incorporated into the crystal lattice at kink sites. As growth proceeds, the surface step winds up in a surface spiral. Often the growth reaction observed occurs in the sequence c, a, b. (From Nielsen, 1964.) (d) Salient features and elementary processes at surfaces.

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