An Introduction to Ionic Solids

When systems crystallize, they adopt the structure whose configuration has the lowest energy crystal structure. In ionic solids, this is mainly determined by the coulombic interaction between the ions: a maximization of the attractive forces between oppositely charged ions and a minimization of the repulsion between like charged ions.

The arrangement of ions in a crystal is known as the crystal structure.
Crystal Structure

The crystal structure is made up of a periodically repeating structural motif known as the unit cell. This is the basic component of the structure, which, when repeated in three dimensions by translational moves only, makes up the complete structure. There may be a choice of unit cells for a given system, such that when repeated give the same overall crystal structure. The one usually adopted as the standard is the one which has the same symmetry as the crystal as a whole.

The pattern of ions or atoms within the structure is given by the lattice. A lattice point need not be a particular ion, but may be a set of ions or atoms, which can be repeated to give the structure. The group of ions associated with a given lattice point are known as the basis for the cell.

Thus if the lattice and the unit cell is known, the entire structure may be elucidated.

In discussing the structures of metals and ionic solids, we generally consider the ions to be spherical objects, or a size given by the ionic radius of the ion, and some of the simplest structures can be understood in terms of the best possible packing arrangements of these spheres. These structures are the ones in which the occupied volume is minimized, and lead to the idea of close-packing of spheres.


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Holes in close packed structures

When we place one layer of spheres on top of another, there are two possible orientations. The hole between layers in position 2 on the diagram above is in the first orientation shown below, and the hole in position 1 on the diagram above is in the second orientation shown below.