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 cubic lattice, the zinc cations are located within tetrahedra of sulfide anions, leading to a coordination number of four.

Only half of all possible tetrahedral ‘holes’ are occupied by zinc cations, leaving the rest empty. The unit cell has four sulfide anions and four zinc cations.

The proportion of cations to anions also affects which structure an ionic solid adopts. Calcium fluoride, which has one cation and two anions, exhibits the fluorite structure.

Here, the cations are arranged in a face-centered cubic lattice with the anions occupying all tetrahedral holes. The unit cell has four calcium cations, each with a coordination number of eight, and eight fluoride anions, each with a coordination number of four.

The antifluorite structure is exhibited by compounds like sodium oxide, which has two cations and one anion. Here, the anions are arranged in a face-centered cubic lattice with cations occupying the eight tetrahedral holes.