\(d=\frac { Z\times M }{ a^{ 3 }\times N_{ A } } \)

\(\Rightarrow 8.92=\frac { 4\times 63.5 }{ 8.92\times 6.02\times 10^{ 23 } } \)
\(\Rightarrow a^{ 3 }=\frac { 4\times 63.5 }{ 8.92\times 6.02\times 10^{ 23 } } \)
\(=\frac { 254 }{ 53.6984\times 10^{ 23 } } \)
\(=4.73\times 10^{ -23 }\)
\(=0.0473\times 10^{ -21 }\)
\(a=0.36\times 10^{ -7 }\ cm\)
For fcc crystal
Radius (r) =\(\frac { \sqrt { 2 } a }{ 4 } \)
\(=\frac { 1.414\times 36\times 10^{ -7 } }{ 4 } \)
\(=0.127\times 10^{ -8 }\ cm\)

When crystals of alkali metal halides such as NaCI are heated in the atmosphere of sodium vapour, the  excess of sodium atoms are deposited on the surface of the crystal. The CI^- ions diffuse to the surface and combine with sodium atoms to form NaCI. This happens by the loss electrons by Na atoms to form the visible light falls on the crystal and occupy anionic sites. As a result, the structure has an excess of sodium.
The crystals acquired yellow colour because when visible light falls on the crystals, they absorb energy from the visible region to excite the electrons. Hence, they are coloured. Similarly, excess of Li makes LiCI crystals pink and excess of K makes KCI crystals viloet.

When imperfections or defects in a crystal are caused by the departure from the definite arrangement in the vicinity of an atom or a group of atoms, the imperfections are called point defects. The imperfections are caused either by missing or dislocations of a constituent particle to a position meant for another particle or shifting to an interstitial position.
Point defects are of two common types:
(i) Schottky defect
(ii) Frenkel defect.
For Schottky and Frenkel defects.

Ferromagnets. The substances which are strongly attracted by the magnetic field and do not lose their magnetism even in the absence of magnetic field are called ferromagnets. For example, Fe₃O₄ .
Ferroelectrics: the substance in which the dipoles may even in the absence of electric field and the direction of polarisation may change by applying electric field are called ferroelectrices. For example barium titanate (BaTiO₃).

2.57\(\times\) 10²¹

6,octahedral

(i) Ferrites are magnetic material. 
(ii) They are used in telephones and memory loops in computers.