Crystal Structure of Zinc Blende

Zinc sulfide crystallizes in two different forms: Wurtzite and Zinc Blende.

The ionic radius of the zinc(II) ion is 0.74 angstroms and that of the sulfide ion is 1.70 angstroms. The ratio of radii for the cation and anion is thus r+/r- = 0.74/1.70 = 0.44.

With a radius ratio of 0.44, one might expect the zinc(II) ions to occupy octahedral holes; however, the value of 0.44 is only slightly larger than rhole/r = 0.414 for an octahedral hole. In this case, the zinc(II) ions occupy tetrahedral holes.

If the sulfide ions originally adopt a hexagonal closest-packed structure, the ZnS crystal is Wurtzite. If the sulfide ions originally adopt a cubic closest-packed structure, the ZnS crystal is Zinc Blende.

The images below depict the structure of Zinc Blende. The yellow spheres represent the sulfide ions and the blue spheres represent the zinc(II) ions.

Examine the images and take note of the following points:

1. The sulfide ions lie in a cubic closest-packed arrangement.

2. The zinc(II) ions are much smaller than the sulfide ions.

3. The insertion of zinc(II) ions into the tetrahedral holes causes the structure to expand so that the sulfide ions are not in contact with each other.

4. Only one half of the tetrahedral holes are occupied by zinc(II) ions. The ionic solid is electrically neutral and the unit cell itself must also be electrically neutral. Because the sulfide ions adopt a ccp structure, there are four sulfide ions in the unit cell. Consequently there must also be four zinc(II) ions in the unit cell. Examine the unit cell and verify this fact.

5. Zinc Blende has (4,4)-coordination.

Full Atoms in the Unit Cell
Portions of Atoms lying in the Unit Cell



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© Copyright 2001, David N. Blauch