What is the total number of tetrahedral voids in $0.6 \ mole$ of a compound that forms a $hcp$ structure?

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?

In a face-centered cubic $(FCC)$ structure,the fraction of the total volume occupied by atoms is .... ($a =$ edge length)

The packing efficiency in a body centered cubic $(bcc)$ structure is closest to $.... \, \%$

Explain three-dimensional close packing from two-dimensional square close-packing. How is the primitive cubic unit cell obtained?

When sodium is heated above $1000 \ K$,its crystal structure changes from $bcc$ to $fcc$. Assuming the metallic radius of the atom remains unchanged,the ratio of the fraction of volume occupied by atoms in $bcc$ and $fcc$ structures will be .......