Fluorite Structures
The Ca+2 ions are arranged in ccp arrangement, i.e. these ions occupy all the corners and the centres of each face of the cube
The F– ions occupy all the tetrahedral holes.
Since there are two tetrahedral holes for each Ca+2 ion and F- ions occupy all the tetrahedral holes, there will be two F- ions for each Ca+2 ions, thus the stoichiometry of the compound is 1:2.
Each Ca+2 ion is surrounded by 8F- ions and each F-ions is surrounded by 4Ca+2 ions. The Coordination number of Ca+2 ion is eight and that of F- ion is four, this is called 8:4 Coordination.
Each unit cell has 4 calcium ions and 8 fluoride ions as explained below
No. of Ca+2 ions = 8(at corners)×1/8 + 6 (at face centres)´1/2
No. of F ions = 8 (within the body)×1 = 8
Thus the number of CaF2 units per unit cell is 4.