Question -
Answer -
(i) Schottky defect
(ii) Frenkel defect
(iii) Interstitials and
(iv) F-centres
Answer
(i) Schottky defect: Schottkydefect is basically a vacancy defect shown by ionic solids. In this defect, anequal number of cations and anions are missing to maintain electricalneutrality. It decreases the density of a substance. Significant number ofSchottky defects is present in ionic solids. For example, in NaCl, there areapproximately 106 Schottky pairs per cm3 at room temperature. Ionic substances containingsimilar-sized cations and anions show this type of defect. For example: NaCl,KCl, CsCl, AgBr, etc.
(ii) Frenkel defect: Ionic solidscontaining large differences in the sizes of ions show this type of defect.When the smaller ion (usually cation) is dislocated from its normal site to aninterstitial site, Frenkel defect is created. It creates a vacancy defect aswell as an interstitial defect. Frenkel defect is also known as dislocationdefect. Ionic solids such as AgCl, AgBr, AgI, and ZnS show this type of defect.
(iii) Interstitials: Interstitialdefect is shown by non-ionic solids. This type of defect is created when someconstituent particles (atoms or molecules) occupy an interstitial site of thecrystal. The density of a substance increases because of this defect.
(iv) F-centres: When the anionic sites of a crystal are occupiedby unpaired electrons, the ionic sites are called F-centres. These unpairedelectrons impart colour to the crystals. For example, when crystals of NaCl areheated in an atmosphere of sodium vapour, the sodium atoms are deposited on thesurface of the crystal. The Cl ions diffuse from the crystal to its surface andcombine with Na atoms, forming NaCl. During this process, the Na atoms on thesurface of the crystal lose electrons. These released electrons diffuse intothe crystal and occupy the vacant anionic sites, creating F-centres.
