In an ionic crystalline solid,atoms of element $Y$ form an $hcp$ structure. The atoms of element $X$ occupy one-third of the tetrahedral voids. What is the formula of the compound?

Vacant space in body centered cubic $(BCC)$ lattice unit cell is about (in $\%$)

Which of the following metals has an $hcp$ crystal structure?

Find the number of tetrahedral voids per atom in a crystal lattice.

In a structure of mixed oxide,oxide ions are in $CCP$. One-fifth of tetrahedral voids are occupied by divalent ion $(A^{2+})$ while half of the octahedral voids are occupied by trivalent ion $(B^{3+})$. The formula of the oxide is:

If $Z$ is the number of atoms in the unit cell that represents the closest packing sequence $ABCABC .....,$ the number of tetrahedral voids in the unit cell is