question_answer

  Explain the following terms with suitable examples. (i) Schottky defect                           (ii) Frenkel defect                                                                                                    [1+1]

Answer:

  Schottky defect A Schottky defect consists of a pair of 'holes' in the crystal lattice. One positive ion and one negative ion are absent. This defect occurs mainly in highly ionic compounds when the positive and negative ions are of a similar size and hence the coordination number is high (usually of 8 or 6). Examples NaCI, CsCI, KCI. Density of the crystal is lowered.                                           [1] (ii) Frenkel defect A Frenkel defect consists of a vacant lattice site (hole) and the ion which ideally should have occupied the site now occupies an interstitial position. This type of defect is favoured by a large difference in size of cations and anions and thus with low coordination number. (4 or 6). Examples ZnS, AgCI, AgBr, and Agl.              [1]