Unit – IX A

Defects in Crystal Structures

Source of Defects:

  • Although crystalline solids have short range as well as long range order in the arrangement of their constituent particles, yet crystals are not perfect.
  • Usually a solid consists of an aggregate of large number of small crystals. These small crystals have defects in them. This happens when crystallisation process occurs at fast or moderate rate.
  • Single crystals are formed when the process of crystallisation occurs at extremely slow rate. Even these crystals are not free of defects.
  • The defects are basically irregularities in the arrangement of constituent particles. The defects are of two types, namely, point defects and line defects.
  • Point defects are the irregularities or deviations from ideal arrangement around a point or an atom in a crystalline substance, whereas the line defects are the irregularities or deviations from ideal arrangement in entire rows of lattice points. These irregularities are called crystal defects.

Point Defects:

  • Point defects are the irregularities or deviations from ideal arrangement around a point or an atom in a crystalline substance.
  • Point defects can be classified into three types :
    (i) stoichiometric defects
    (ii) impurity defects and
    (iii) non-stoichiometric defects.

Stoichiometric Defects:

  • These are the point defects that do not disturb the stoichiometry of the solid. They are also called intrinsic or thermodynamic defects.
  • These defects are of two types, vacancy defects and interstitial defects.

    Vacancy Defect:

    When some of the lattice sites are vacant, the crystal is said to have vacancy defect.

    This defect can also develop when a substance is heated.

    The defect produced due to vacancies caused by an absence of anions and cations in the crystal lattice of ionic solid is called a Scottky defect.

    Due to this defect, the observed density of crystal is found to be lower than expected density.

    This defect is observed in solids with cations and anions have almost equal size. like NaCl, KCl, CsCl etc.

Interstitial Defect :

When cation and anion from ionic solid leaves its regular site and moves to occupy a place between the lattice site is called interstitial defect. This defect associated with ionic compound is called Frenkel defect or dislocation defect.

This defect is observed in case of relatively smaller cations which can fit into interstitial space and hence produce the defect.

The presence of this defect does not alter density of the solid.

This defect is common when the difference in ionic radii of two participating ions is large.

Example: In AgCl the defect is observed due to Ag+ ions. In ZnS the defect is observed due to Zn++ ions.

Impurity Defect:

  • This defect occurs when regular cation of a crystal is replaced by some different cation. The different cation may occupy regular lattice site or interstitial site.
  • If the impurity cation is substituted in place of regular cation then the defect is known as substitution impurity defect. If the impurity cation occupies interstitial space then the defect is called interstitial impurity defect.
  • Interstitial impurity in Stainless Steel
  • Substitutional impurity in Brass
  • Substitutional impurity in NaCl

If molten NaCl containing a little amount of SrCl2 is crystallised, some of the sites of Na+ ions are occupied by Sr2+ . Each Sr2+ replaces two Na+ ions. It occupies the site of one ion and the other site remains vacant.

The cationic vacancies thus produced are equal in number to that of Sr2+ ions.

  • Another similar example is the solid solution of CdCl2 and AgCl.

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