Translation dislocations

Examples of defect lines created in cholesterics according to the Volterra process are shown in Fig. 17. The core structure will be considered below this being replaced by a narrow cylinder. Figures 17 a and b represent translation dislocations in both cases. 

Fig. 33) seen in smectic A phases. Focal curves and translation dislocations are not excluded from fan textures. The fan patterns themselves can be absent from certain arrangements of disclinations, which however belong to the family of fan textures. This is illustrated by the rhombus in Fig. 34 a and b, where two -i- r and two -tt disclinations are associated with several focal curves at the top and bottom levels of the mesophase. 

As indicated above and illustrated in Fig. 32 i, layers lie horizontally in planes and show some translation dislocations. They are oblique in polygonal fields and generally cross the focal lines at points where they are strongly hat-shaped, whereas elsewhere they are saddle-shaped and dislocations are present, often superimposed on the focal lines. In fans, the layers are vertical and all types of defect (disclinations, focal curves, dislocations) are present. 

True walls are defined by a director discontinuity extending over a surface within a liquid crystal, but these defects are excluded from nematic, cholesteric and smectic A phases (see Sec. 7.2.3). Grain boundaries comparable to those of true crystals are associated with translation dislocations, which lie parallel within the wall or form... 

Translation dislocations do not strongly modify the orientation of the directors or layers at long distances, as disclinations in liquid crystals do, and focal conics occupy an intermediate position (see Fig. 32 i). This indicates a gradation of energies, which will be considered below using some examples. 

All the expressions given above for discli-nation and focal curve energies are similar to those for screw and edge dislocations in true crystals [9,15], but the situation is different for translation dislocations in smectics. The presence of a screw dislocation of weak Burgers vector does not introduce any splay and does not really modify the layer thickness even at small distances [16]. The energy is reduced to a core term and therefore interaction terms are absent in the frame of the linear theory. This is not the case for edge dislocations in this approximation [106], and one has the expression ... 

As in crystals, defects in liquid crystals can be classified as point, line or wall defects. Dislocations are a feature of liquid crystal phases where tliere is translational order, since tliese are line defects in tliis lattice order. Unlike crystals, tliere is a type of line defect unique to liquid crystals tenned disclination [39]. A disclination is a discontinuity of orientation of tire director field. 

The other major defects in solids occupy much more volume in the lattice of a crystal and are refeiTed to as line defects. There are two types of line defects, the edge and screw defects which are also known as dislocations. These play an important part, primarily, in the plastic non-Hookeian extension of metals under a tensile stress. This process causes the translation of dislocations in the direction of the plastic extension. Dislocations become mobile in solids at elevated temperamres due to the diffusive place exchange of atoms with vacancies at the core, a process described as dislocation climb. The direction of climb is such that the vacancies move along any stress gradient, such as that around an inclusion of oxide in a metal, or when a metal is placed under compression. 

Figure 4.1 Schematic dislocation line a simple cubic crystal structure. The line enters the crystal at the center of the left-front face. It emerges at the center of the right-front face. The shortest translation vector of the structure is the Burgers Vector, b. The line bounds the glided area of the glide plane (100) from the unglided area.

Wilkinson BM, Tyson JR, Reid PJ, Stirling CJ (2000) Distinct domains within yeast Sec61p involved in post-translational translocation and protein dislocation. J Biol Chem 275 521-529... 

In the case of anthracene, the stable monoclinic phase transforms under stress to a triclinic phase in which molecules are favourably oriented for dimerization to occur. Although the triclinic phase has not been isolated as a pure phase, its structure has been established using low-temperature electron microscopy and atom-atom potential calculations (Jones Thomas, 1979). In l,8-dichloro-9-methyl anthracene, isolated dislocations with (201) [010] translation bring the molecules to the required geometry (Fig. 8.17) to facilitate photodimerization. 1, 5-dichloroanthracene is an interesting case. Instead of the expected 100% head-to-head dimers, photoreaction yields 80%... 

Accordingly, glide planes are those planes which have the shortest b vectors a/2 <110> for fee, a/2 <111> for bcc, and a/3 <211.0) for hep lattices. Dislocations can split into so-called Shockley partials b = bx +b2, if b2>b +b. Since b and b2 are not translational vectors of the crystal lattice, they induce a stacking fault. The partial dislocation therefore bounds the stacking fault. 

Another class of dislocations called partial dislocations is characterized by Burgers vectors corresponding to a translation of an atom from a lattice position to a twin-lattice position. Such dislocations are related to stacking faults in close-packed crystal structures and appear at the boundaries of stacking faults ending inside a crystal [5.49]. 

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