Ionic Crystal Structures

Ionic solids typically are closely packed in a crystal lattice. However, the charges and sizes of the ions greatly influence their packing configuration, leading to significant variation in crystal structure and coordination number.

For example, caesium chloride, where both ions are of similar sizes, resembles a primitive cubic lattice of chloride anions with caesium cations in the centers. The caesium chloride unit cell is assigned one chloride anion and one caesium cation.

Each caesium ion is surrounded by eight chloride ions and vice versa, giving both caesium and chloride coordination numbers of eight.

Sodium chloride resembles a face-centered cubic lattice of chloride anions with the smaller sodium cations occupying the spaces between the anions.

The difference in ionic radii results in each sodium cation interacting with only six chloride anions and vice versa, with four chloride anions and four sodium cations assigned to the unit cell. This is often referred to as the rock salt structure.

Zinc sulfide has an even greater difference in ionic radii than that of sodium chloride, leading to the zinc blende structure. Although the sulfide arrangement resembles a face-centered cub