March 3, 1999
This is the wurtzite crystal structure. Each tan ball is a cadmium atom; each orange ball is a selenium atom. The sticks between them represent bonds. The two atoms have opposite charges (plus and minus), thus each dumbbell has a dipole moment vector (remember that dipole moment is measured as the magnitude of the charge on the atoms times the distance between them). Since the dumbbells are assembled in the same orientation, the entire structure has a dipole moment.
Here is an example of the zincblende crystal structure. It is also assembled from dumbbells assembled in the same orientation. For a better understanding of each structure, you can click on the image. You will be able to manipulate and view the structures in three dimensions.
The Chime plugin is needed to view these structures. If you have the plugin, then proceed and enjoy. If not, bookmark this page and download the Chime plugin from http://www.mdli.com.