IMPERFECTIONS IN SOLIDS

IMPERFECTIONS IN SOLIDS

IDEAL CRYSTAL : An ionic crystal has the same unit cell containing same lattice points throughout the whole of the crystal. It exists only at absolute zero (0K).

IMPERFECTIONS : It is the disorder from perfectly ordered state of constituents of crystals.

DEFECTS : are of two types –

1.) Point Defects – Defects at a particular lattice points.

2.) Line Defects – Defects in a complete row of a crystal lattice.

Three types of Point Defects : Stoichiometric defects, Non-Stoichiometric defects and Impurity defects

1.) STOICHIOMETRIC DEFECTS : The defects which donot effect the stoichiometric or formulae of a compound or a crystal. Stoichiometric  compounds are those in which no. of (+) and (-) ion are exactly in the ratio as indicated by their chemical formulae. It is of four types :

(a) Vacancy Defect : Some constituent particles are missing from their normal positions.

(b) Interstitial Defect : Some constituent particles are present in the vacant interstitial sites of a crystal lattice.

(c) Schottky Defect : It arises when no. of anions and cations are equally missing in a crystal lattice holes or vacancies. Density of a crystal gets decreased by it. Eg.- NaCl, CsCl, AgBr.  ( In NaCl, 10 schottly pairs per cc. are present ).

Causes : High coordination no. ; Cations and anions are of almost similar sizes.

(d) Frenkel Defect : When an ion (cation) is missing from its normal position and occupies an interstitial site b/w lattice points but the no. of anions and cations remain same, i.e., crystal is electrically neutral. It doesnot effect the density of a crystal. Eg.- AgCl, ZnS, AgBr.

Causes : Low CN; Anions are larger in size than cations.

2.) IMPURITY DEFECT : The cation gets replaced by another cation of similar size and high charge. Eg.- In Na  ions are replaced by Sr   ions, i.e., 2 Na  ions = 1 Sr   ions such that crystal remains electrically neutral. Also, doping of CdCl2 in AgCl.

3.) NON-STOICHIOMETIC DEFECT : The defect in which the ratio of (+) and (-) ions present in a compound becomes different from that required by chemical formulaes. It is of two types :

(a) Metal Excess Defect : (+) ions are in excess.  – Anion Vacancies ; - Excess cations

(i) Anion Vacancies : In this defect, anions may be missing from their lattice sites leaving holes in which electrons remain entrapped to maintain electrical neutrality. Eg. – When alkali metals are heated in the atmosphere of alkali metal vapors, anion vacancies are produced. Metal atoms get deposited on the surface of alkali metal halide crystal and halide ions diffuse out from the crystal to combine with metal atoms  leaving behind their electrons at anion vacancies known as F-centres. It causes violet colour in KCl, pink or lilac colour of LiCl, yellow colour in NaCl.

(ii) Excess Cations Occupying interstitial sites : In this defect, there are extra cations occupying interstitial sites and same no. of electrons in another interstitial sites to maintain electrical neutrality. Eg. – ZnO loses oxygen at high temperature and turns yellow in colour.

(b) Metal Deficient Defect : anions are in excess. – Cation Vacancies ; - Excess anions

(i) Cation Vacancies : Cations may be missing from their lattice sites. The extra (-) charge may be balanced by some nearby metal ion acquiring 2 (+) charges instead of 1. Eg.- In FeS, Fe   is absent or missing and 2Fe   ions are converted into Fe   to maintain electrical neutrality. As a result, crystal has metallic lusture nd iron pyrite is named as Fool’s Gold.

(ii) Extra Anions Occupying Interstitial Sites : In this case, extra anions occupy interstitial sites and extra (-) charge is balanced by extra charges on adjacent metal ions.