Dislocations in Nonmetallic Crystals

Dislocations occur profusely in nonmetallic materials. As mentioned above, ceramics are brittle at ordinary temperatures, not because of a lack of dislocations but because these cannot easily glide due to strong bonding between the component atoms. Organic crystals, which are usually composed of molecules consisting of strongly bound atoms, linked by weak external bonds, usually glide by movement of molecules rather than atoms, and dislocations can be referred to the molecular array rather than the atom array. 

Many ceramic crystals can be described in terms of close-packed planes of oxygen atoms (or ions). For example, corundum, AI2O3, can be regarded as consisting of layers of oxide ions, O2-, with small Al3+ ions contained in an ordered fraction of 

In an ionic material such as sodium chloride the dislocation can bear a charge, which will depend upon the half-planes inserted. If the tip has an excess of cations, it will be positively charged whereas if the tip consists of anions, it will be negatively charged (Fig. 3.19b). Similarly, when the dislocation reaches the surface, a charge may be present, which may enhance chemical reactivity at this 

Burgers vector b, [101], is the shortest vector connecting two identical atoms in this structure. (.b) An edge dislocation in the NaCl structure, consisting of two extra half-planes of atoms. If these are ionic, the tip will bear a charge, depending upon the ions in the termination, and slip will occur on 110.  

This is one mechanism by which dislocations can become decorated (Section 3.7.3). A classic example is the decoration of dislocations in potassium chloride, 

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