Solid State Test - 7

Let us calculate contribution of 8 atoms at the corners (8 x 1/8 = 1), and 6 at the face centres (6 x 1/2 = 3), giving a total of 4 atoms per unit cell.

There are many examples of ccp (fcc) (ABCABC) metal structures. For example, Al, Ni, Cu, Ag, Pt.

As 'A' atoms are present at the 8 corners of the cube, so number of atoms of A in unit cell = 1/8 × 8 = 1. 

As 'B' atoms are present at the face centres of the cube, so number of atoms of B in unit cell = 1/2 × 6 = 3. Hence, the formula of the compound is AB₃.

At room temperature, elements Li, Na, K, Rb, Ba, V, Cr and Fe have structures that can be described as body-centred cubic (bcc) packing of spheres. The general formula of an ionic compound crystallising in body-centred cubic structure is AB.

The unit cell present in ABAB closest packing of atoms is hexagonal.

The general formula of an ionic compound crystallising in rock-salt structure is AB.

The method of studying crystals using X–rays is Bragg's spectrometer method.

An ion leaves its regular site and occupies a position in the space between the lattice sites. This defect is called Frenkel defect.

Frenkel defect is present in those compounds which have low coordination number and large difference in the size of cations and anions. Hence, due to small size of Zn²⁺ ion and large size of S^2- ion, ZnS shows this defect.