Critical configuration:

• In order to reduce potential energy each ion tends to surround itself with as many ions of opposite sign as possible.

• This tendency results in the formation of close packed structures. This can also be stated as “The ions try to maximize their coordination with neighbouring ions.

• In ionic crystals coordination number is defined as “The number of nearest anions surrounding a central cation. This is also called the Ligancy. The factor which controls the Ligancy is ionic packing or packing geometry. As we know energy is given off as atoms/ions are brought closer together, until equilibrium distance is reached. The solid becomes more stable if atoms/ions are closely packed and average interatomic distance is reduced.

• From this it is clear that size of atoms/ions is the important factor for the packing efficiency and stability limit.

• Consider a case of coordination number 3 i.e. three surrounding anion (shown by hollow circles) and one cation (shown by dark circle) as shown in Figs. 1(a), (b) and (c).

• In Fig. 1(a), the cation (dark circle) is loosely fitted into the space between three anions (hollow circles). Here repulsive forces among the anions are more than the attractive forces between cation and anion. This makes the situation unstable. Similarly in Fig. 1(c) three anions are in touch with one cation and there is no enough space to fit another anion that will touch cation. This configuration is stable but not quite stable configuration.