Introduction to Polyhedral Models
An alternative to represent ionic compounds is to use polyhedral models. When we can see the way in which three=dimensional shapes represent the individual units, it helps visualize long range order. The crystal structure model becomes unwieldy when you represent long range order in the form of multiples of unit cells.
The Two Basic Polyhedrons
The two most common shapes are octahedrons and tetrahedrons, Figure 1. Cations occupy the center of a polyhedron and anions take vertices.
Other shapes exist, but these two suffice for now. Then you can see the character of long range order often results from how polyhedral shapes connect to one another.
Polyhedrons Have More Than One View
Either one of these polyhedral models can have more than one way to depict them, Figure 2.
The stereo view shows a three-dimensional form turned in a three quarter angle. The shading indicates the direction of the faces.
Octahedral forms also have an end on view. This shows an octahedron looking down a top axis. The octahedron rests on one of its triangular faces.
The inverted view of a tetrahedron represents a tetrahedron turned up onto an edge with an opposing edge perpendicular to the first edge.
Sodium Chloride (Halite) Example of Polyhedral Models
Sodium chloride acts as a representative example of how a crystal structure can also be shown as a combination of polyhedral models. The crystal structure of NaCl has chloride ions at the vertices of a cube with occupied by chloride ions. Smaller sodium cations sit in each octahedral interstitial site.
There is no specific reason to choose sodium chloride other than the fact it is a simple crystal structure which is reasonably easy to understand.
Relationship of Polyhedrons to Crystal Structure
Superimposing the shape formed by an octahedron onto a sodium chloride lattice results in an image which shows the relationship of lattice point models to polyhedral models, Figure 3.
In the lattice, green spheres represent chloride anions and red spheres denote sodium cations. Each sodium atom has six chlorine atoms as its nearest neighbor.
The polyhed formed with six atoms distributed around a single central sodium cation forms an octahedron.
The red spheres represent a sodium cation while green atoms show chloride atoms.
How Octahedra Fit Together
Octahedra can share part of their atoms with neighboring octahedron in one of three ways:
each octahedra shares a vertices with an adjacent nearest neighbor join octahedra at each one of its lattice points
octahedra attach to its neighbors through two of its atoms with an edge shared between each adjoining octahedra
three atoms are shared between each octahedra joined by its neighbors by its triangular face
Polyhedral Models Compared to Crystal Structure of NaCl
The side by side views of the crystal lattice model and the polyhedral model show how the two of models together provide a more complete concept of the structure of NaCl. Figure 5 shows the crystal structure of halite compared to the a polyhedral model of NaCl.
Built Up Polyhedral Models of NaCl
Building up a polyhedral model has two possibilities: the polyhedral models viewed from the side or viewed from the end looking down, Figure 6.
The depiction on the left shows a stereo view of sodium chloride where the octahedra cluster has been built from octahedra with a three-quarter aspect.
On the right a top down view shows how polyhedral models reveal an otherwise difficult to visualize a feature present in the long range order of NaCl. Between the octahedral polyhedrons a tetrahedral hole results.
Example Tetrahedral Polyhedral Models
Sphalerite (Zinc Sulfide)
Zinc sulfide adopts one of two common structures: sphalerite and Wurtzite. Figure 7 shows sphalerite adjacent to a space filling model built from tetrahedral polyhedral models.
The crystal structure is composed of chloride ions which occupy the lattice points of a face center cubic close packed structure. Zn+2 ions sit in every other tetrahedral hole.
Polyhedral models form from a Zn+2 ion at the center of a tetrahedron while S-2 ions reside at each vertex. The tetrahedral shape connect together by sharing S-2 ions in a corner sharing scheme.
Many Polyhedral Models
Other ionic compounds adopt a variety of polyhedral models. These simple examples serve to illustrate how to understand these kind of representations. Between crystal structures and polyhedral models, a more complete concept of ionic structures provide enhanced insight.
Many more structures exist, which will be elaborated when needed. This should provide a start the next time polyhedral models are encountered.