Question Topics :
That is why packing of metals and ceramics are different. In metals, atoms are considered having same size while because in ceramics like NaCl, cation and anion have different sizes, anions being bigger form the base lattice while cations occupy the interstices. In the latter case, one needs to follow the Pauling's rules. Please check Module 1 for detailed discussion on ionic ceramics.
In case of simple cubic for single elemental material, atoms are positioned at the corners only and all corner positions are equivalent as lattice is repeatable or periodic in space and hence position (100) or (010) or (001) or (111) is identical as (000). While in case of BCC/FCC, since these are non-primitive lattices, there are two identical positions (000) and (1/2,1/2,1/2) in BCC while in four positions in FCC are (000), (1/2,1/2,0), and (1/2,0,1/2) and (0,1/2,1/2) which are translatable by appropriate lattice vectors.
This is because BCC is not a closed packed structure and atomic arrangement is such that the planes on which octahedral voids are forms are more closely packed than those found for tetrahedral voids. Hence, it results in larger tetrahedral voids than octahedral, which sounds bizarre but is true.