Edge dislocations planar defects

Dislocations Dislocations are stoichiometric line defects. A dislocation marks the boundary between the slipped and unslipped parts of crystal. The simplest type of dislocation is an edge dislocation, involving an extra layer of atoms in a crystal (Fig. 25.2). The atoms in the layers above and below the half-plane distort beyond its edge and are no longer planar. The direction of the edge of the half-plane into the crystal is know as the line of dislocation. Another form of dislocation, known as a screw dislocation, occurs when an extra step is formed at the surface of a crystal, causing a mismatch that extends spirally through the crystal. 

Figure 1.1 Defects in crystalline solids (a) point defects (interstitials) (b) a linear defect (edge dislocation) (c) a planar defect (antiphase boundary) (d) a volume defect (precipitate) (e) unit cell (filled) of a structure containing point defects (vacancies) and (/) unit cell (filled) of a defect-free structure containing ordered vacancies. ...

The lattice defects are classified as (i) point defects, such as vacancies, interstitial atoms, substitutional impurity atoms, and interstitial impurity atoms, (ii) line defects, such as edge, screw, and mixed dislocations, and (iii) planar defects, such as stacking faults, twin planes, and grain boundaries. 

The appearance of a (5-7-7-5) defect can be interpreted as the nucleation of a degenerate dislocation loop in the planar hexagonal network of the graphite sheet. The configuration of this primary dipole is a (5-7) core attached to an inverted (7-5) core. The (5-7) defect behaves thus as a single edge dislocation in the graphitic plane. Once... 

Figure 16. Creation of defects in a smectic A phase, (a, a ) Creation of a right-handed screw dislocation, (b, b ) Creation of an edge dislocation, (c, c ) Creation of a stack of nested conic layers, as observed along focal conics, (d, d, d") Creation of a disclination from a planar cut surface limited by a line L a +jt separation of lips S] and S2 is followed by the addition of matter and relaxation.

This texture distribution is common, but corresponds to a rather schematic model, indicating that high-energy defects, such as disclinations, are found mainly in the vicinity of the isotropic transition. Other situations are observed in thick preparations of cholesterics, for example, where planar domains can be interrupted by walls of vertical layers (Fig. 34 c and d), due to edge dislocations disjoining into disclina-tion pairs (see Fig. 5 d). Despite these par-... 

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