Ionic compounds generally have more complicated structures than metals. This is probably because:

(1) There are now at least two kinds of particles in the lattice, generally of different sizes.

(2) The cations attract the anions, but like ions repel one another. The structure must balance both types of forces.

(3) Many ions (e.g. nitrate, carbonate, azide) are very non-spherical in shape. They will thus pack differently in different directions.
One simple ionic structure is:

Cesium Chloride
Cesium chloride crystallizes in a cubic lattice.  The unit cell may be depicted as shown.
(Cs is teal, Cl- is gold).
Click on the unit cell above to view a movie of the unit cell rotating.
 When these unit cells are stacked into a lattice, they form a structure such as below.
Click on the  images below to view the cesium lattice rotating.
horizontal           vertical


Notice that you cannot see through the structure; there are no channels.
However, the tightly-packed structures make it difficult to view the interior relationships.
The same structure, but with the ions moved further apart allows the interior to be viewed.
Click on the images below to view the open structure rotating.
horizontal          vertical

One way to describe the crystal is to consider the cations and anions separately. We begin with the larger (gold colored) Cl- ions. Ignoring the Cs+, we note that the Cl- themselves form a simple cubic anion sublattice. 

Considering only the Cs+, they form a simple cubic cation sublattice.

This animation shows the CsCl lattice, only the teal Cs+ (the Cs sublattice), and only the gold Cl- (the Cl sublattice).

We can describe the CsCl structure as two interpenetrating simple cubic cation and anion sublattices.
(The two sublattices are not necessarily the same, although in this example they are.)

Recall that the simple cubic lattice has large interstitial sites between each  8 atoms. We can therefore think of making the CsCl by taking a simple cubic Cs lattice and placing Cl into the interstitial sites.

This is obvious if we compare the CsCl unit cell with the simple cubic unit cell showing the interstitial site.

We may also consider the neighborhood in which each ion finds itself. Each Cs+ is surrounded by 8 Cl- (so the Cs+ coordination number is 8) at the corners of each cube.

Each Cl- is also surrounded by 8 Cs+ at the corners of a cube, so the Cl- has CN = 8.

Click here to go to the next page.
 Structure of Crystals
Crystal Lattices
Unit Cells
From Unit Cell to Lattice
From Lattice to Unit Cell
Packing & Geometry
Simple Cubic Metals
Close Packed Structures
Body Centered Cubic
Cesium Chloride
Sodium Chloride
Rhenium Oxide
Niobium Oxide
Except as otherwise noted, all images, movies and VRMLs are owned and copyright  by
Dr. Barbara L. Sauls and Dr. Frederick C. Sauls  1998.
Contact the owners for individual permission to use.   barbarasauls@kings.edu