Imperfections Defects in Ceramics

When the periodic arrangement of a crystal is interrupted, a deviation from a perfect and orderly arrangement of the array of the lattice points generally occurs. Such a deviation from the periodic arrangement may be localized, in the 

Pelleg, Mechanical Properties of Ceramics, Solid Mechanics and Its AppUcations 213, DOl 10.1007/978-3-319-04492-7 3, Springer Intenational Publishing Switz land 2014 

Although a point defect may be considered to be a volume defect of atomic dimensions, in this section the common term point defect will be used. Impurity atoms are often also considered to be point defects. Perhaps a solute atom at a substitutional or interstitial site should also be considered as an imperfection, namely as a point defect, since there is a deviation from the original periodicity of the pure crystal. However, an impurity atom is present in a crystal unintentionally, whereas solute atoms are purposely added to a pure material. Generally, atoms are added to pure materials to enhance certain properties, mechanical or physical. Regarding mechanical properties, bear in mind that some impurities may strengthen a material, but others may be detrimental to its mechanical properties. Clearly, the common interest is to enhance the mechanical properties of a given material. 

The basic, structural point defects in very pure crystals are the vacancies and the interstitials, the former representing a vacant lattice site, while the latter is an extra atom at a non-lattice site. Either one of them is highly localized and characterized, as mentioned above, by the disturbance around a single atomic site. A perfect crystal is thermodynamically stable only at absolute zero temperature. At any higher temperature, the crystal must contain a certain number of point defects. For example, it is probable that an atomic site is vacant at low temperature, i.e., a vacancy is only 10 , whereas, at the melting point, this probability is 10 . Thus, point defects are a thermodynamic feature, unlike other defects such as line defects. 

Various internal and external surfaces, such as grain boundaries and stacking faults [henceforth SF], are considered as two-dimensional defects and they comprise the class known as planar defects. 

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