Question

In fluorite structure, $C{a}^{2+}$ are present at FCC positions. Which of the following is correct?

• A. Edge length $\left(a_{fcc}\right)=2r^{+}+2r^{-}$
• B. Coordination number of cation $=8$
• C. Coordination number of anion $=6$
• D. Tetrahedral voids occupied by anion $=50\%$

Answer 1

The correct option is B Coordination number of cation $=8$

In flourite structure, the $C{a}^{2+}$ ions are present at the FCC sites and the $F^{-}$ ions are present in all the tetrahedral voids,
For edge length $\sqrt{3}a=2r_{+}+4r_{-}$
The tetrahedral voids can be seen as the centres of the minicubes formed by dividing a unit cell into 8 equal parts by cutting each side by half.
The tetrahedral voids are present on the body diagonals.
Coordination number of cations in flourite structure is 8 because it is surrounded by the anions present at the tetrahedral voids.
Coordination number of anions in flourite structure is 4 because anions are present in the tetrahedral voids.
All the tetrahedral voids are occupied by the anions .