Question

Anti fluorite structure is derived from fluorite structure by:
(A) Heating fluorite crystal lattice
(B) Subjecting fluorite structure to high pressure
(C) Inter-changing the positions of positive and negative ions in the lattice
(D) None of the above

Answer 1

Hint: Fluorite structure is a common structure of the compounds with the formula $M{X}_{2}$. The X occupies the tetrahedral sites and M occupies the regular or ccp in the FCC structure. Anti fluorite is a structure that is identical with that of fluorite but has some difference in its structure with respect to the structure of fluorite.

Complete answer:
The anti fluorite structure is derived from the fluorite structure by interchanging the position of positive and negative ions in the lattice. And thus, the correct option is (c).

• In the fluorite structure, let us take an example, $Ca{F}_{2}$, the positive ion is ${Ca}^{2+}$ and the negative ion is ${F}^{-}$. It has a coordination number of 8:4.
• In the above structure, the ${Ca}^{2+}$ ions occupy the ccp and the ${F}^{-}$ occupy all the tetrahedral voids.
• But, as we know to derive the anti fluorite structure the positive ions are interchanged with the negative ions. Therefore, the coordinate number becomes 4:8.
• An example of an anti fluorite structure is ${Na}_{2}O$.
• In ${Na}_{2}O$, ${Na}^{+}$ is the positive ion and ${O}^{2-}$ is the negative ion.
• In this structure, the ${O}^{2-}$ ions occupy the ccp and the ${Na}^{+}$ ion occupy all the tetrahedral voids.

Note: Fluorite crystallizes in a cubic motif. It has four perfect cleavage planes that help in producing octahedral fragments. The motif structure that is adopted by the fluorite is so common that the motif itself is called the fluorite structure.