We should now be able to see the rationale for the additional absences in BCC and FCC lattices ( relative to an SC lattice ).
For a BCC lattice, x = y = z = 1/ 2 since an atom is present at x = y = z = 1/ 2 a where a = edge length. In such a situation, the phase difference between the A and B layers is ( h + k+ l )
. This means that when the value of h + k + l = odd, there is destructive interference and all reflections with h + k + l = odd are absent in a BCC lattice. Our analysis so far has assumed identical atoms in the A and B layers.
Even if the atoms in the adjacent layers have very similar scattering powers, the same absences would occur. In general, the scattering power of an atom or an ion depends on its electron density distribution (r) where (r) is given by (r) =
( r ) (r) ) (see Module 1).
The scattering power of an atom A, fA is related to A (r) by
. This means that when the value of h + k + l = odd, there is destructive interference and all reflections with h + k + l = odd are absent in a BCC lattice. Our analysis so far has assumed identical atoms in the A and B layers. 
(r) where
( r ) 
(r) ) (see Module 1). 
