Defects in Solids - Practice Questions & MCQ

Point Defects, Line 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 a large number Of small crystals. These small crystals have defects in them. This happens when the crystallisation process occurs at a fast or moderate rate. Single crystals are formed when the process of crystallisation occurs at an extremely slow rate. Even these crystals are not free of defects. The defects are basically irregularities in the arrangement of constituent particles. Broadly speaking, the defects are of two types, namely, point defects and line defects. Point defects are irregularities or deviations from ideal arrangement around a point or an atom in a crystalline substance, whereas the line defects are irregularities or deviations from ideal arrangement in entire rows of lattice points. These irregularities are called crystal defects. We shall confine our discussion to point defects only.

Types of Point Defects

• Stoichiometric Defects
  Those compounds in which the number of positive and negative ions are exactly in the ratio indicated by their chemical formula are called stoichiometric compounds example, NaCl. These solids show following types of defects:

  1. Vacancy Defect: When some of the lattice sites are vacant. the crystal is said to have a vacancy defect.
     
     
     This results in a decrease in the density of the substance. This defect can also develop when a substance is heated.

  2. Interstitial Defect: When some constituent particles (atoms or molecules) occupy an interstitial site, the crystal is said to have an interstitial defect.
     
     
     This defect increases the density of the substance. Vacancy and interstitial defects as explained above can be shown by non-ionic solids. Ionic solids must always maintain electrical neutrality. Rather than simple vacancy or interstitial defects, they show these defects as Frenkel and Schottky defects.

  3. Frenkel Defect: This defect is shown by ionic solids. The smaller ion (usually cation) is dislocated from its normal site to an interstitial site.
     
     

     It creates a vacancy defect at its Original site and an interstitial defect at its new location. Frenkel defect is also called dislocation defect. It does not change the density of the solid. Frenkel defect is shown by ionic substance in which there is a large difference in the size of ions, for example, ZnS, AgCl, AgBr and Agl due to small size of Zn²⁺ and Ag⁺ ions.
      
  4. Schottky Defect: It is basically a vacancy defect in ionic solids. In order to maintain electrical neutrality, the number of missing cations and anions are equal.
     
     
     Like simple vacancy defect, Schottky defect also decreases the density of the substance. Number of such defects in ionic solids is quite significant. For example, in NaCl, there are approximately 10⁶ Schottky pairs per cm³ at room temperature. In 1 cm³ there are about 10²² ions. Thus, there is one Schottky defect per 10¹⁶ ions. Schottky defect is shown by ionic substances in which the cation and anion are of almost similar sizes. For example, NaCl, KCl, CsCl and AgBr. It may be noted that AgBr shows both, Frenkel as well as Schottky defects.

• Impurity Defects
  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.
  
  

• Non-Stoichiometric Defects
  There are many compounds in which the ratio of positive and negative ions present in the compound differs from the required by ideal formula of the compound. Such compounds are called Non-stoichiometric compounds. For example, VO_x 
  In these compounds, a balance of positive and negative charges is maintained by having extra electrons or extra positive charge. These defects are of following types:

  1. Metal Excess Defect

     • Due to Anionic Vacancies and (F-centres)
       
       
       A compound may have excess metal ion if a negative ion is absent from its lattice site, leaving a hole which is occupied by an electron to maintain electrical neutrality.

       1. The holes occupied by electrons are called F -centres and are responsible for the colour of the compound.

       2. The excess of sodium in NaCl makes the crystal to appear yellow.

       3. Excess of potassium in KCI makes it violet.

       4. Excess of lithium in LiCl makes it pink.

       5. Greater the number of F-centres greater is the intensity of colour. This type of defects is found in a crystal which is likely to possess Schottky defects.

     • Due to Cationic Vacancies
       
       
        

       1. It may occur if an extra positive ion is present m an interstitial site.

       2. Electrical neutrality is maintained by the presence of an extra electron in the Interstitial site.

       3. These types of defects are exhibited by the crystals which are likely to exhibit Frenkel defects. Example,— Yellow colour of ZnS.

  2. Metal Deficiency Defect
     The non-stoichiometric compounds may have metal deficiency due to the absence of a metal ion from its lattice site. The charge is balanced by an adjacent ion having higher positive charge. These type of defects are generally shown by compounds of transition metals. For example, FeS, NiO.