An ionic crystal which has the same unit cell constituting the same lattice points throughout the whole of crystal is known as ideal crystal. However, such ideal crystals exist only at absolute zero (0 K) temperature.
Any deviation from perfectly ordered arrangement of constituent particles in crystal is called imperfection or defect. The defect may also arise due to the heat absorbed by the crystals from the surrounding or due to the presence of impurities in the crystals.
There are commonly two types of imperfections:
- Electronic imperfections
- Atomic imperfections or point defects.
The perfectly ionic or covalent crystals at 0 K have electrons present in the fully occupied lowest energy states. But at higher temperature (above 0 K), some of the electron may occupy higher energy states, depending upon the temperature, then the defect arises is called electronic imperfections.
In crystals of pure silicon, some electrons are released thermally from covalent bonds at temperature above 0 K. These electrons become free to move and, therefore, they are responsible for the electrical conductivity. The bonds from which electrons have been removed become electron deficient and these are referred to as holes.
When the deviations or irregularities exist from the ideal arrangement around an atom in a crystalline substance, the defect is called point defect or atomic defect
However, when the deviation from the ideal arrangement exists in the entire row of lattice points, the defect is called line defect or dislocation.
The point defects in ionic crystals may be classified as:
- Defects in stoichiometric crystals.
- Defects in non-stoichiometric crystals.
- Impurity defects.
1.: Defects in Stoichiometric Crystals
Stoichiometric compounds are those in which the numbers of positive and negative ions are exactly in the ratios indicated by their chemical formulae.
[ i.e., NaCl = 1 : 1 , CaCl2 = 1 : 2 , Na2O = 2 : 1 ]
In these compounds two types of defects are generally observed.
These are: (a) Schottky defect and (b) Frenkel defect
In an ionic crystal, if equal number of cations and anions are missing from their lattice sites by leaving behind cation vacancy and anion vacancy so that the electrical neutrality is maintained, it is called Schottky defect.
In this defect, the density of the crystal markedly lowered. This defect was discovered by German scientist Schottky in 1930
This defect is observed in strongly ionic compounds having:
- High coordination number
- Ions (cations and anions) having almost similar sizes.
Example: NaCl, KCl, KBr and NaBr ionic solids have Schottky defects.
NOTE: In NaCl, there are about 106 Schottky pairs per cm3 at room temperature. In 1 cm3 there are about 1022 ions and this means that there will be 1 Schottky defect per 1016 ions in NaCl.
In an ionic crystal, when an ion is missing from its normal position and occupies an interstitial site between lattice points is called Frenkel defect or interstitial defect. The crystal remains electrical neutral because total charge on cation and anion remains same.
This defect is usually observed in compounds having:
- Low coordination number
- Anions are much larger in size than the cations
Examples: AgCl, AgBr, AgI, ZnS solids have frenkel defects. This is due to small size of Ag+ ion and Zn+2 ion, these ions can go into the interstitial sites.
[AgBr show both Schottky and Frenkel defects]
2.: Defects in Non-Stoichiometric Crystals
Non-stoichiometric compounds are those in which the numbers of positive and negative ions are different in the ratios as indicated by their chemical formulae but crystal remains neutral.
Non-stoichiometric behavior is most commonly found for transition metal compounds.
A.) Metal Excess Defect:
In these defects, the positive ions are in excess than anions. The compounds having metal excess defect may also refer as n-type semiconductor. These may arise due to the following two ways: (i.) Anionic vacancies and (ii.) Presence of extra cation in interstitial sites.
In this case, anions may be missing from their normal lattice sites leaving behind anionic vacancies in which the electrons remain entrapped to maintain electrical neutrality.
The electrons trapped in anion vacancies are referred to as F-centers (from German word farbenzenter meaning colour center). They impart yellow colour to the crystals of NaCl. Similarly, KCl appear violet and LiCl crystal appear pink due to F-centers.
This type of defect is observed in those crystals which are likely to form Schottky defects.
Presence of Extra cation in Interstitial sites
In this case, there are extra positive ions occupying interstitial sites and the electrons in another interstitial sites to maintain electrical neutrality. The defect may be visualised as the loss of non-metal atoms which leave their electrons behind.
This type of defect is found in crystals which are likely to develop Frenkel defect.
Example: Zinc oxide, it is white in colour at room temperature. On heating it loses oxygen reversibly at high temperature and turns yellow in colour.
Zn+2 ions are trapped in interstitial sites and equal numbers of electrons are trapped in the neighborhood to balance the electrical charge.
B.) Metal Deficient Defect:
In this defect, the number of cations is less than number of anions. This defect occurs when the metal ion shows variable oxidation state. The compounds having metal deficient defect may refer as p-type semiconductor. These arise due to two ways: (i.) Cation vacancies (ii.) Extra anion occupying interstitial sites
In some cases, the positive ions may be missing from their lattice sites. The extra negative charge may be balanced by some nearby metal ion acquiring higher oxidation state.
Example: Ferrous oxide (FeO), Ferrous sulphide (FeS), Nickel oxide (NiO) etc.
In case of iron pyrites (FeS), two out of three ferrous ions in lattice may be converted into Fe+3 state and the third Fe+2 ion may be missing from its lattice site. As a result, the crystal has metallic lustre. Because of the natural colour of iron pyrites and metallic lustre some samples of minerals shine like gold so have been nick-named as fool’s gold.
Presence of Extra Anion in Interstitial sites
In this case, the extra anion may be occupying interstitial positions. The extra negative charge is balanced by acquiring extra charges on the adjacent metal ions. Such type of defect is not common because the negative ions are usually very large and they cannot easily fit into the interstitial sites.
3.: Impurity Defect in Ionic Crystals
These defects in ionic crystals arise due to the presence of some impurity ions at the lattice sites or at the vacant interstitial sites.
Example: If molten NaCl containing a little amount of SrCl2 is allowed to crystallize, some of the sites of Na+ ions are occupied by Sr+2 ions. For each Sr+2 ion introduced, two Na+ ions are removed to maintain electrical neutrality.
[Note: Similar defect and behaviour is observed when CdCl2 is added to AgCl.]
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