In ccp structure, there are 8 tetrahedral voids. In dose-packed structure, there are eight spheres in the corner of the unit cell and each sphere is in contact with three others giving rise to eight tetrahedral voids.

In a ccp structure, there is 1 octahedral void in the centre of the body and 12 octahedral voids are on 12 edges of the cube.Each octahedral void on the edge is

common to four other unit cells (as shown in figure). Thus, in cubic close-packed structure, octahedral voids in the centre of the cube = 1.
Effective number of octahedral voids on edges = 12 x 1/4 = 3
Total number of octahedral voids = 1 + 3 = 4

Y = 8 ×\(\frac{1}{8}\)= 1; X = 1,
∴ Formula is XY.

Structure of alkali metal halides: At ordinary temperature and pressure,halides of alkali metals (iodides, bromides and chlorides of lithium, sodium, potassium and rubidium) posses NacI type structure with 6:6 coordination.
When high pressure is applied, the coordination number increases and the alkali metal halides transform to CsCI structure with coordination number 8:8.

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).

For a fcc structure, length of the unit cell is related to radius of atom as
\(r=\frac { a }{ 2\sqrt { 2 } } \)
\(\\ or \ a=2\sqrt { 2 } r\)
\(=2\times 1.414\times 127.8 \ pm=361.42pm\)
\(Density, \ \rho =\frac { Z\times M }{ { a }^{ 3 }\times { N }_{ A } } \)
\(=Z=4(for \ fcc),M=63.55 \ g \ { mol }^{ -1 }\)
\(a=361.42\times { 10 }^{ -12 }m=361.42\times { 10 }^{ -10 }cm\)
\({ N }_{ A }=6.02\times { 10 }^{ 23 }\)
\(\rho =\frac { 4\times 63.55 }{ { \left( 361.42\times { 10 }^{ -10 } \right) }^{ 3 }\times 6.02\times { 10 }^{ 23 } }\)
\(=8.94 \ g{ cm }^{ -3 }\)
\(Density, \ \rho =\frac { Z\times M }{ { a }^{ 3 }\times { N }_{ A } } \)
\(a=286.65pm=286.65\times { 10 }^{ -12 }m\)
\(=286.65\times { 10 }^{ -10 }cm\)
\(\ Z=2(for \ bcc), \ M=56.0\)
\(\rho =7.87 \ g{ cm }^{ -3 }\)
\(7.87=\frac { 2\times 56.0 }{ { \left( 286.65\times { 10 }^{ -10 } \right) }^{ 3 }\times { N }_{ A } }\)
\( { N }_{ A }=\frac { 2\times 56.0 }{ { \left( 286.65\times { 10 }^{ -10 } \right) }^{ 3 }\times 7.87 } \)
\(=6.04\times { 10 }^{ 23 }\)

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