Which crystal lattice has the maximum packing efficiency?

$A$ compound is formed from elements $X$ and $Y$. The atoms of $Y$ (anions) form a $ccp$ lattice. The atoms of $X$ (cations) occupy half of the octahedral voids and half of tetrahedral voids. What is the formula of the compound?

In a solid,oxide ions are arranged in $HCP$. Cations $A$ occupy one-sixth of $THV$ and cations $B$ occupy one-third of the $OHV$. The empirical formula of the compound is

Explain the formation of voids during the transition from a two-dimensional hexagonal close-packed layer to a three-dimensional close-packed structure.

$A$ compound is formed by two elements $A$ and $B$. Atoms of the element $B$ (as anion) make $ccp$ lattice and those of element $A$ (as cation) occupy all tetrahedral voids. The formula of the compound is

$A$ metal crystallizes in a lattice having $AB AB AB$ sequence of layers. This arrangement of spheres corresponds to which type of packing,and what is the percentage of empty space (void volume) in this lattice?