question_answer 47)

Explain the following terms with suitable examples : (i) Schottky defect                          (ii) Frenkel defect (iii) Interstitials                                (iv) F-centres.  

Answer:

(i) Schottky defect. This type of defect is created when equal number of positive and negative ions are missing from their respective positions leaving behind holes. Since the number of missing positive ions is equal to the number of missing negative ions the crystal as a whole is electrically neutral. This defect is more common in ionic compounds with high coordination number and where the ions (positive and negative) are of similar size. For example NaCI, KCI, CsCI and KBr. Since the number of ions decreases, therefore as a result of large number of Schottky defects in solid, the density of solid decreases. (ii) Frenkel defect This type of defect is created when an ion leaves its lattice site and occupies an interstitial site. In this case the crystal remains electrically neutral because the number of anions and cations remain the same. Since there is no absence of ions from the lattice, the density remains the same. Frenkel defects generally occur in ionic compounds (i) which have low co-ordination number, and (ii) in which anions are much larger in size than the cations. These defects can be found in silver halides (such as AgCI, AgBr or Agl) because of small size of the Ag+ ion, it can go into the interstitial sites. (iii) Interstitials. Interstitial sites are the holes or voids in the crystals. Atoms (or ions) which occupy the vacant interstitial positions in a crystal are called Interstitials. (iv) F-centres (Farbe's centre or colour centres) F-centres are the free electrons trapped in the anionic vacancies which are responsible for colour and electrical conductance in non-stoichiometric compounds. e.g., When sodium chloride is heated in an atmosphere of sodium vapours, the excess of metal atoms get deposited on the surface of alkali metal crystal. Halide ions then diffuse to the surface where they combine with metal ions. The electrons so produced by the ionisation of the metal diffuse back into the crystal and occupy anion vacancy. These electrons absorb some energy of the white light, giving yellow colour to NaCl. These amionic sites occupied by unpaired electron are referred to as F-ccntres (German : Farbezenter means colour centre).