We readily note that systematic absences can be used to identify the nature of the unit cell.
To understand the absences, we need to consider the phase differences between reflections from adjacent layers when the atoms in the layers are of different type. The case where layer 1 is A, layer 2 is B and layer 3 is A again is shown in Fig 19.2
Figure 19.2 Diffraction patterns from three layers A , B and A showing the phase differences in reflections from different layers.
The distance between the two A layers is a and the distance between the A and B layers is xa. When there is a constructive interference from two adjacent A layers, the phase difference between the layers is 2
. For an analogous reflection from the B layer, the phase difference is 2
x as the distance a is reduced by a factor of x. Considering all the indices h k l, the phase difference hkl between adjacent h k l planes when the distances between the planes are xa, yb and xc is given by
hkl = 2 ( h x + k y + l z )
(19.2)
Note that when x, y, z are 1, the phase difference corresponding to a, b and c gives the maximum intensity because 2 a, 2 b and 2 c together correspond to 2 d sin = n ) . These phase differences correspond to 100 or 010 or 001 type reflections. If the reflection is from 200 planes, then the phase difference will be 4 x and not 2 x.
. For an analogous reflection from the B layer, the phase difference is 2
x as the distance a is reduced by a factor of x. Considering all the indices h k l, the phase difference 
) . These phase differences correspond to 100 or 010 or 001 type reflections. If the reflection is from 200 planes, then the phase difference will be 4 
