Question

In an anti-fluorite structure, cations occupy

• A. corners of the cube
• B. tetrahedral voids
• C. centre of the cube
• D. octahedral voids

Answer 1

The correct option is B tetrahedral voids

Anti-fluorite structure (example: $N{a}_{2}O$) is an inverse of fluorite structure (example: $Ca{F}_2$ where cations form a regular CCP structure with anionic fluoride ions occuping all tetrahdral sites).
In antifluorite structure, anions form CCP structure while the tetrahedral sites are occupied by the cations.
Each cation at tetrahedral void is surrounded by 4 oxide anions and the oxide anions are surrounded by 8 cations, each.
Hence, the coordination number of cation is 4 and coordination number of anion is 8.
The alkali metal oxides $M_{2}O(M=Li,Na,K,Rb)$ crystallize in an anti-fluorite structure.