Solid State Test - 7

Conduction of charge is due to movement of charged particles, i.e. electrons or ions that are available and free to move. In case of ionic solids, although the ions are present but they are not free to move. However, the constituent ions in the ionic compounds acquire mobility, in the molten state or in their aqueous solutions.

For the orthorhombic crystal system:
a = b = c; α = β = γ = 90°

NaCl is an example of cubic crystal system.

In a body-centered cubic unit, cell points are present at all the corners as well as at the centre of the cube.

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.

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.

Each minicube has 4 S^2- ions. So, each Zn²⁺ has4 nearest neighbours.

CsCl has a BCC,structure, with Cl^- ions at the corners and Cs⁺ at the body centre.
Coordination number of each ion is 8.

Either Na⁺or Cl^- may be chosen as lattice point. Choosing Cl^- ions, one sees that the simplest cubic arrangement contains a chloride ion in the centre of each face as well as at the corners of the unit cell. Hence, the cell is face-centred cubic.
There are 8 corners, 8 x 1/8 = 1 Cl^- ion, at the 6 faces, 6 x 1/2 = 3 Cl^- ions, along the edges, 12 x 1/4 = 3 Na⁺ ions and at the cube centre, 1 Na⁺ ion, thereby making a total of 4 Na⁺ and 4 Cl^- ions. Thus, the unit cell contains 4 formula units of NaCl.