Engineering Physics

2.10 NaCl crystal structure

The Bravais lattice of NaCl crystal is a face-centred cube as shown in Fig. 2.16. NaCl is an ionic crystal. It consists of two FCC sub-lattices, one of Cl⁻ ions having origin at (000) and the other of Na⁺ ion having origin at one half of the body diagonal of the unit cell. The Na⁺ and Cl⁻ ions are present at equilibrium distances in the crystal. The attractive force between Na⁺ or Cl⁻ ions in the crystal is balanced by the repulsive forces between them due to same type of charges. In this crystal, either Na⁺ or Cl⁻ ions could occupy the corner and face-centre positions with the ions of the opposite type occupying the alternate positions in between them. Each ion is surrounded by six other type ions so that the co-ordination ...