5Schotkky DefectsIn in ionic crystal of the type A+ B-, equal number of anionsand cations are missing from the lattice sites so thatelectrical neutrality is maintained it is called schottky defect.Types of compounds exhibiting SchottkyDefect This type of defect is shown by highly ioniccompounds which have High coordination number Cations and anions of similar sizes
6Consequences of Schottky Defect As the number of ions decrease, volume isthe same, so density decreases. Crystalswith Schottky defect conduct electricity to asmall extent. Due to the presence of holesthe stability of the crystal decreases.
7Frenkel Defect If an ion is missing from its lattice site (causing a vacancy or a hole there) and it occupies theinterstitial site so that electrical neutrality as well asstoichiometry of the compound are maintained.This type of defect is called Frenkel defect.
8Types of compounds exhibiting Frenkel defect This type of defect is present in those compounds which have Low co-ordination number. Large difference in size of anion and cation. Examples: AgCl, AgBr, AgI and ZnS.
9ConsequencesSolids with this defect conduct electricity to a small extent The dielectric constant of the crystal increases The density of the solid is unchanged Due to the presence of holes, the stability of the crystal decreases
10Interstitial Defects Atoms or ions, which occupy normally vacant interstitial positions in a crystal, are calledinterstitials. The important factor determiningthe formation of interstitials is the size of theatom/ion, because they are accomodated in thevoids.
11Non-Stoichiometric Defects If an imperfection causes the ratio of cations toanions to become different from that indicated bythe ideal chemical formula, the defect is callednon-stoichiometric.
12Metal Deficiency due to anion vacancies A negative ion may be missing from its lattice site, leaving a hole, which is occupied by an electron thereby maintaining an electrical balance. The trapped electrons are called F-centers or color centers because they are responsible for imparting color to the crystal.
13By presence of extra cations in the interstitial sites Extra cations occupying interstitial sites with electrons present in another interstitial site to maintain electricalneutrality can cause metal excess. This defect is similar to Frenkel defect and is formed in crystals having Frenkeldefects. Example: If ZnO is heated, it loses oxygen andturns yellow.
14Metal Deficiency due to cation vacancies In case of ionic solids, the impurities are introduced by addingimpurity of ions. If the impurity ions are in a different oxidation state fromthat of the host ions, vacancies are created. For e.g., If molten NaCl, containing a little of SrCl2 as impurity isallowed to cool, in the crystals of NaCl formed, at some lattice sites Na+ions are substituted by Sr2+ ion. For every Sr2+ thus introduced twoNa+ ions are removed to maintain electrical neutrality.