Ionic Radii:
     Ca²⁺, 99 pm
     F ^-, 136 pm
Radius Ratio = 99/136 = 0.73

The radius ratio is on the borderline between octahedral and cubic coordination. Experiment shows that Ca²⁺ ions occupy cubic holes.

The stoichiometry (one Ca²⁺ ion per two F ^- ions) requires that half of the cubic holes be occupied. In fact, the CaF₂ structure consists of a simple cubic array of F ^- ions with Ca²⁺ ions filling half of the cubic holes.

Click on the image at right to view the Calcium Fluoride structure. The links in the text above go to definitions.