An ionic compound is formed by elements $X$ and $Y$. Element $Y$ forms a face-centered cubic $(FCC)$ lattice, and element $X$ occupies one-third of the tetrahedral voids. What is the formula of the compound?

Packing fraction of a layer in $2D$ hexagonal close packing arrangement is:

Consider the $bcc$ unit cells of the solid $1$ and $2$ with the position of atoms as shown below. The radius of atom $B$ is twice that of atom $A$. The unit cell edge length is $50\%$ more in solid $2$ than in $1$. What is the approximate packing efficiency in solid $2$? $........... \%$

In a crystal lattice,the number of octahedral voids compared to tetrahedral voids is ...............

Which of the following is the correct order of packing efficiency?

Element '$B$' forms $ccp$ structure and '$A$' occupies half of the octahedral voids,while oxygen atoms occupy all the tetrahedral voids. The structure of bimetallic oxide is