Ionics solids are stable, high-melting-point substances held together by strong electrostatic forces between oppositely charged ions.

The structures of ionic solids are formed by the packing of spheres. The three types of holes in the packed structures include:

• Trigonal holes, formed by three spheres of the same layer
• Tetrahedral holes, a sphere sits in the dimple of three spheres in an adjacent layer
• Octahedral holes, two sets of three spheres in adjoining layers
• Holes increase in size in this order:

      Trigonal < Tetrahedral < Octahedral