Schematic Illustration of Point Defects

An atom of a lattice site may become displaced to an interstitial position, as shown in Fig. 3.2. 

The displaced atom may be an atom of the crystal and, in that case, the term self- interstitialcy is used, or it may be a foreign atom in the structure, located at an interstitial site. In Fig. 3.2, a self-interstitial or self-interstitialcy is shown. 

To summarize. Fig. 3.5 presents all the above point defects in one figure, showing the possible configurations in ceramics. 

Both these kinds of vacancies may occur in ceramics, but, only cation interstitials are likely to form, because anion interstitials are too large and the very heavy distortion prevents such a defect formation. 

As mentioned in the introduction, impurities are considered as point defects. There are no perfect crystals in addition to point defects, they also contain a large number of various impurities. 

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