Solid State Test - 3

For fcc, the octahedral voids are located at centre of cube and the mid point of edges. The tetrahedral voids are located at 1/4 distance from the corners on each body diagonal. 
Considering the void at centre, surrounded by 8 tetrahedral voids, all at a distance of 3a/4 within the same unit cell. These have to be nearest voids, because any tetrahedral void in a neighbouring cell has to be at a larger distance(nearest point in a neighbouring cell is at a/2).

In fcc unit cell, 

a2=4r 

r=a2/4 =4002/4 

=141.4pm 

For octahedral void, 

2(r+R)=a 

2R=a-2r 

=400-2*141.4=117.16 pm 

So, the diameter of greatest sphere=117.16 pm

r⁺/r^- =126/216 =0.58 

This corresponds to fcc structure, so The coordination number of Ag in AgI is 6.

For BCC lattice, 2(r⁺ + r^-)=3a 

r⁺=(3*480)-225 =190.7 pm

Frenkel defect occur due to movement of smaller cations into interstitial space due to larger difference in ionic radii of cations and anions. 

Frenkel defect is also known as dislocation defect.

Solids which do not have an ordered arrangement of their constituent ions but have a disordered arrangement are called amorphous solids. They lack long range order which is a characteristic of crystal.

Electrolytes are substances that conduct electricity in the molten state or in an aqueous solution. By melting or dissolving in water, ions are free to move around. As a result, the solution can be made to conduct electricity.