Test: Imperfections in Solids - NEET

. (a) A Schottky defect consists of a pair of holes in the crystal lattice. One positive ion and one negative ion are absent.

Note This sort of defect occurs mainly in highly ionic compounds where cation and anion are of similar size, hence the coordination number is high (8.6) as in NaCI, CsCI, KCI, KBr

The picture is shown as cation excess that means metal excess with the absence of anion.

 (a) If there is no deficie ncy of cation or anion but cation migrates to interstitial site, it is called dislocation. It is observed in the crystals with smaller cation and larger anion, so that cation can dislocate easily. It is called Frenkel defect.

. (a) Frenkel defect is observed in the crystals in which cation is much smaller than the anion so that cation can migrate to interstitial position.

(a)  Non-stoichiometric compounds are also called Berthollide compounds thus, correct.

(b) Since, composition of the constituent elements is not fixed, thus law of constant composition is not followed as is Fe 0,98 O thus, correct.

(c) If cation (say A⁺) is missing then, charge is balanced by the cation A²⁺. If anion (say B^-) is missing, then charge is balanced by inserting electron (e^-)thus, correct.

 (a) For fcc structure

 = 0.414 for octahedral void

= 0.225 for tetrahedral void 

Thus, maximum packing can be for octahedral void

Metal oxide = M_0.98O
If ‘x’ ions of M are in +3 state, then
3x + (0.98 – x) × 2 = 2
x = 0.04
So the percentage of metal in +3 state would be

 (c)

Due to the addition of SrCI₂, each Sr²⁺ ion replaces two Na⁺ ions, but occupies one Na⁺ lattice point. Thus, this exchange of Na+ ion by Sr²⁺ ion makes one cationic vacancy.

SrCI² doped = 10^-3 mol per 100 mol = 10^-5 mol per 1 mol

∴ Cation vacancies = 10^-5 mol per 1 mol

= 10^-5 x N₀ mol^-1

= 10^-5 x 6.02 x 10²³

Total = 6.02 x 10¹⁸ cationic vacancies mol^-1  

(a, b) AgBr is a covalent c om pound with smaller A g⁺ (= 115 pm) and larger Br-(≈196 pm) thus, Ag⁺ ion can be migrated to interstitial site showing Frenkel defect. In AgBr, there is also Schottky defect per10¹⁶ ions.

In Substitution impurity, foreign atom takes the position of parent atom. Toatal number of atoms remains same after the defect.

Strength of metal is inversely proportional to size of grain. Coarse are softer in nature while material having fine grain structure are hard in nature.

. (a,b,c,d)

(a) Since, number of cations > number of anions, hence it I represents metal-excess defect. It is similar to Schottky defect i in which equal number of cations and anions are missing | thus, correct.

(b) Charge is balanced by electrons, these are called F-centres (farbe, colour producing). Compounds with F-centres are ! coloured. Greater the number of F-centres, greater the ! intensity thus, correct.

(c) Due to the unpaired electrons these are paramagnetic thus, correct. j:

(d) When NaCI, KCl, etc., are strongly heated in flame, they become deficient in anions and represented as AS, _ s where, § is small fraction.
Non-stoichiometric form of NaCl is yellow and that is KClis blue-lilac thus, correct

(a) correct
(b) As cation (A' ) occupies interstitial position, thus cation is smaller than anion, thus correct.
(c) Due to the smaller cation and larger anion, coordination number Is usually low (4, 6), thus correct.
(d) By Fajans' rule, smaller cations polarise larger anions thus, compounds are covalent, thus correct. 

 (a, b)

(a) When cation, anion or both are missing, molar mass is y ass decreased. Since, density = Mass/ Volume. hence density Volume decreases, thus correct.

(b) When there is insertion of cation (A + ) in the interstitial site, it causes metal-excess defect. Thus, molar mass increases, hence density increases, thus correct.

(c) Presence of impurity increases molar mass (as doping of metal with P or Si), hence density changes, thus incorrect.

(d) In Frenkel defect, cation is dislocated without affecting molar mass thus, no change in density , thus correct

(b)

AB is rock salt like structure, thus number of AB molecule per
unit cell = 4, a = 2y^1/3 nm

Obseved value of d(20 kgm^-3) >>5kgm^-3

Thus, there is excess of metal ions which incerease mass hence the density

AB is rock salt like structure, thus number of AB molecules per unit cell  =4, a = 2y^1/3 nm

Thus, there is excess of metal ions which increases mass hence the density. 

Rock salt (NaCl) is fcc type structure, z = 4

M = 64 g mol-1, a = 418 pm

theoretical density (d) = 

= 5.82 g cm^-3

Experimental density = 4.92 g cm^-3

thus, experimental density << theoretical density

thus, there is vacancy in the crystal

d= Mass / Volume

Due to decrease in density, molar mass should decrease. this indicates Schottky defect.

Let Schottky defect be x%

Rock salt (NaCl) is fcc type structure, z = 4

M = 64 g mol-1, a = 418 pm

theoretical density (d) = 

= 5.82 g cm^-3

Experimental density = 4.92 g cm^-3

thus, experimental density << theoretical density

thus, there is vacancy in the crystal

d= Mass / Volume

Due to decrease in density, molar mass should decrease. this indicates Schottky defect.

Let Schottky defect be x%

(i) When cation or anion is absent, molar mass decreases hence, density decreases.
Thus, (i) - » (q)

(ii) When cation is packed in the interstitial site, molar mass increases. Hence, density increases.
Thus, (ii) - » (s)

(iii) When smaller cation migrates into the interstitial site, Frenkel defect is observed. There is no change in molar mass and thus, no change in density.
Thus, (iii) -> (p)

(iv) As in NaCI, when cation and anion are missing (Schottky defect), molar mass and thus, density decreases.
Thus, (iv) - 0