Question

Which of the following is TRUE for Flourite type structure?

• A. Cations are present at tetrahedral voids
• B. It has 8:4 coordination.
• C. Anions are present at octahedral voids
• D. It has 4:8 coordination

Answer 1

Answer: (A), (B)

It has 8:4 coordination.

Just recall the structure of $Ca{F}_2$ here. $Ca{F}_2$ is an example of fluorite type structure only.

The calcium ions, $C{a}^{2+}$ , are located at the face-centered cubic lattice points and, therefore, have cubic closed-packed arrangement. The fluoride ions ($F^{⊝}$ ions) occupy all the 8 tetrahedral voids. Thus, there are 4 $C{a}^{2+}$ ions and 8 $F^{⊝}$ ions in the unit cell of calcium fluoride.

Since each $C{a}^{+2}$ surrounded by 8 $F^{⊝}$ ions and

each $F^{⊝}$ is surrounded by 4 $C{a}^{+2}$ ions.

Thus, the structure has 8 : 4 coordination

So, only (a) and (b) are correct.