Calcium fluoride occurs naturally as the mineral fluorite.
The ionic radius of the calcium ion is 1.26 angstroms and that of the fluoride ion is 1.17 angstroms. In this case the cation is the larger ion, and the ratio of radii for the anion and cation is r-/r+ = 1.17/1.26 = 0.929. The two ions are essentially the same size; thus it makes little difference whether one talks about inserting fluoride ions into holes in a calcium ion lattice or vice versa.
With a radius ratio of 0.929 (essentially unity), the smaller ion is expected to prefer a cubic hole.
The images below depict the structure of Fluoride. The light blue spheres represent the fluoride ions and the red spheres represent the calcium ions. The top two images illustrate the cubic holes occupied by calcium ions; however, these two images do not show the unit cell. The bottom two images show the fluorite unit cell.
Examine the images and take note of the following points:
1. The fluoride ions lie in a cubic arrangement.
2. The calcium and fluoride ions are comparable in size.
3. The insertion of calcium ions into the cubic holes causes the structure to expand so that the fluoride ions are not in contact with each other.
4. The top two images, which show a simple cubic cell, do NOT depict the unit cell for fluorite. Because calcium fluoride contains two fluoride ions for every calcium ion, it is not possible to fill all cubic holes with a calcium ion, as this would violate electroneutrality for the structure. In Fluorite, half of the cubic holes contain a calcium ion and the other half are empty. The bottom two images show the unit cell for Fluorite. Examine the bottom two images to verify the 2:1 fluoride ion:calcium ion stoichiometry.
5. The Fluorite structure actually contains calcium ions in a cubic closest-packed structure with fluoride ions in a tetrahedral environment. In the ccp structure, the unit cell contains four atoms and there are eight tetrahedral holes, all of which are filled by fluoride ions in Fluorite. Examine the bottom two images and verify this structure.
6. Fluorite has (8,4)-coordination.
| Full Atoms in the Fluorite Structure |
Portions of Atoms lying in the Fluorite Structure |
| Full Atoms in the Unit Cell |
Portions of Atoms lying in the Unit Cell |
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