Imperfections in solids

A perfectly ordered crystal can exist only at absolute zero. Above this temperature some departure from ordered arrangement occurs. These are called imperfection or defects.Crystals contain a group of micro crystals with irregularities at crystals boundaries or grain boundaries. These are called lattice defects. The irregularities may also be present at several lattice points. They are called points defects.
Point defects
These are defects localized at lattice points or at voids. Thus 1 cm3 of a crystal of NaCl with 10^22 Na+ and 10^22Cl- ions has 10^6 vacant Na+ lattice positions at room temperature. Metallic crystals also contain such vacant lattice positions due to missing of metal atom in the lattice. The lattice positions from which lattice atoms or ions are missing are called vacancies. Thus ionic crystals have cation and anion vacancies. Point defects can also arise due to various other factors and it is convenient to classify them as follows.
1. Stoichiometric defects
2. NonStoichiometric defects
3. Impurity defects
1. Stoichiometric defects
They are defects, which do not affect the stoichiometry or formula of the compound.These are further divided in to two.
a. Schottky defects
These are defects in ionic crystals, which arise due to missing of equal amounts of cations and anions. A cation vacancy is compensated by an anion vacancy of equal charge. Thus NaCl,KCl,CsCl,KBr etc contain this defect.
The empirical formula in unaffected but the density of the crystal is lowered and electrical conductance is increased by such defects. These defects are predominant in ionic crystals with cation and anion of nearly equal size.

b. Frenkel defects
In ionic crystal such as those like AgCl the cations(Ag+) are quite small compared to the anions(Cl-). Some cations move away from lattice positions giving rise to a cation vacancy. The cation now occupies an interstitial void such as octahedral or tetrahedral voids. Such defects are termed Frankel defects. Here the empirical formula and the density are unaffected but conductance increases.
2. Non Stoichiometric point defects
These are point defects, which affect the formula of the compound. They are of two types
a. metal deficiency defects
b. metal excess defect

a. Metal deficiency defect
Ferrous oxide is expected to have the formula FeO. But the actual formula is in the range Fe0.95O. The metal deficiency is due to missing of some Fe2+ ions from the lattice. To maintain electrical neutrality double the number of Fe2+ acquire higher oxidation state of Fe3+.
b. Metal excess defects
Such crystals have higher proportion of cation than required by stoichiometric formula. Thus NaCl crystal heated in Na vapour condense on the crystal surface and ionize to Na+ ion and electron both Na+ and electron diffuse in to the crystal lattice. Na+ ion occupy a cation vacancy and electron in a anion vacancy. The anion vacancy which has trapped the electron is called an F center or colour center.
3. Impurity defects
Point defects can be introduced in solids by addition of impurities. Thus if NaCl crystals are grown in a saturated solution of NaCl containing some SrCl2. To maintain electrical neutrality for every Na+ displaced by Sr2+ one Na+ will be missing from the lattice.

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